Method, synthesis, activation procedure and characterization of an oxygen rich activated porous carbon sorbent for selective removal of carbon dioxide with ultra high capacity

ABSTRACT

The present disclosure pertains to materials for CO 2  adsorption at pressures above 1 bar, where the materials include a porous material with a surface area of at least 2,800 m 2 /g, and a total pore volume of at least 1.35 cm 3 /g, where a majority of pores of the porous material have diameters of less than 2 nm as measured from N 2  sorption isotherms using the BET (Brunauer-Emmett-Teller) method. The present disclosure also pertains to materials for separation of CO 2  from natural gas at partial pressures of either component above 1 bar, where the materials include a porous material with a surface area of at least 2,200 m 2 /g, and a total pore volume of at least 1.00 cm 3 /g, where a majority of pores of the porous material have diameters of greater than 1 nm and less than 2 nm as measured from N 2  sorption isotherms using the BET method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patent application Ser. No. 15/200,632, filed on Jul. 1, 2016, which claims priority to U.S. Provisional Patent Application No. 62/187,744, filed on Jul. 1, 2015. The entirety of each of the aforementioned applications is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND

Current materials for capturing carbon dioxide (CO₂) suffer from numerous limitations, including limited CO₂ sorption capacity and selectivity. Various embodiments of the present disclosure address these limitations.

SUMMARY

In some embodiments, the present disclosure pertains to materials for CO₂ adsorption at pressures above 1 bar. In some embodiments, the materials include a porous material with a surface area of at least 2,800 m²/g, and a total pore volume of at least 1.35 cm³/g. In some embodiments, a majority of pores of the porous materials have diameters of less than 2 nm as measured from N₂ sorption isotherms using the BET (Brunauer-Emmett-Teller) method.

In some embodiments, the present disclosure pertains to materials for the separation of CO₂ from natural gas at partial pressures of either component above 1 bar. In some embodiments, the materials include a porous material with a surface area of at least 2,200 m²/g, and a total pore volume of at least 1.00 cm³/g. In some embodiments, a majority of pores of the porous materials have diameters of greater than 1 nm and less than 2 nm as measured from N₂ sorption isotherms using the BET (Brunauer-Emmett-Teller) method.

In some embodiments, the porous materials of the present disclosure include a porous carbon material with a carbon content of between 80% and 95% as measured by X-ray photoelectron spectroscopy. In some embodiments, the porous materials of the present disclosure include a porous carbon material with a surface area of at least 2,800 m²/g, a total pore volume of at least 1.35 cm³/g, and a carbon content of between 80% and 95% as measured by X-ray photoelectron spectroscopy.

In additional embodiments, the present disclosure pertains to materials for the separation of CO₂ from natural gas at partial pressures of either component above 1 bar. In some embodiments, the materials include a porous carbon material with a surface area of at least 2,000 m²/g, a total pore volume of at least 1.00 cm³/g, and a carbon content of greater than 90% as measured by X-ray photoelectron spectroscopy.

In some embodiments, the porous carbon materials of the present disclosure are prepared by heating an organic polymer precursor or biological material in the presence of potassium hydroxide (KOH). In some embodiments, the temperature of activation is between 700° C. and 800° C. In some embodiments, the temperature of activation is between 600° C. and 700° C.

In some embodiments, the porous carbon materials of the present disclosure are prepared by heating an organic polymer precursor. In some embodiments, the organic polymer precursor includes oxygen in a functional group. In some embodiments, the functional group is a furyl. In some embodiments, the organic polymer precursor is furfuryl alcohol. In some embodiments, the organic polymer precursor polymerizes to form polyfurfuryl alcohol (PFFA). In some embodiments, PFFA is prepared by the polymerization of furfuryl alcohol with a catalyst. In some embodiments, the catalyst is iron(III) chloride.

In some embodiments, the functional group is an anisyl. In some embodiments, the organic polymer precursor is anisyl alcohol (AA). In some embodiments, the organic polymer precursor polymerizes to form polyanisyl alcohol (PAA). In some embodiments, PAA is prepared by the polymerization of AA with a catalyst. In some embodiments, the catalyst is a protic acid.

In some embodiments, the porous carbon materials of the present disclosure are prepared by heating a biological material. In some embodiments, the biological material includes, without limitation, sawdust, coconut husk, and combinations thereof. In some embodiments, the biological material is chosen from at least one of the following: sawdust and coconut husk.

Additional embodiments pertain to methods of making the materials of the present disclosure. Further embodiments pertain to utilizing the materials of the present disclosure for the capture of CO₂ from various environments.

DESCRIPTION OF THE FIGURES

FIGS. 1A-1B provide data relating to high pressure CO₂ uptake (at 30 bar and 24° C.) as a function of the surface area (FIG. 1A) and total pore volume (FIG. 1B) for a range of porous carbons (PCs), including N-containing PCs (NPCs) and S-containing PCs (SPCs). Also shown are data relating to high pressure CO₂ uptake at different pressures as a function of surface area (FIG. 1C) and total pore volume (FIG. 1D). A comparison of the total pore volume as a function of surface area for a range of PCs, NPCs and SPCs is also shown (FIG. 1E).

FIG. 2 provides a plot of CO₂ uptake at 30 bar and 24° C. as a function of activation temperature for PC, NPC, and SPC samples.

FIGS. 3A-3B provide estimated surface area (FIG. 3A) and total pore volume (FIG. 3B) as a function of activation temperature for PC, NPC and SPC samples.

FIG. 4 provides comparative data relating to CO₂ uptake as a function of CO₂ pressure on N-containing polymer polypyrrole (PPy) precursors and PPy precursors activated at different temperatures (PPy-T-2). Sorption measurements were performed at 24° C.

FIG. 5 provides N₂ adsorption isotherms for four different NPC samples of PPy-T-2 prepared from polypyrrole and activated at the labelled temperature (T). Sorption measurements were performed at 24° C.

FIGS. 6A-6B provide data relating to the determination of pore structures by N₂ physisorption isotherms of PPy-T-2 samples activated at different temperatures by N₂ physisorption isotherms. Shown are the estimated surface area (FIG. 6A) and total pore volume (FIG. 6B) versus activation temperature.

FIGS. 7A-7H show additional related data for PPy-T-2 samples. FIG. 7A shows the pore size distributions of PPy-T-2 samples prepared at the three activation temperatures shown, as determined by the non-local density functional theory (NLDFT) method. FIG. 7B shows a scanning electron microscope (SEM) image of a PPy-600-2 sample. FIG. 7C shows high resolution transmission electron microscope (HRTEM) images of the PPy-600-2 sample. FIG. 7D summarizes high pressure volumetric CO₂ adsorption uptake measurements on PPy-800-2 and PPy-800-4, showing the effect of the KOH:precursor ratio. Sorption measurements were performed at 24° C. Also shown are typical X-ray photoelectron spectroscopy (XPS) survey scans for the polypyrrole precursor (FIG. 7E) and PPy-600-2 NPC samples (FIG. 7F). Also shown are the wt % determined by XPS of elemental carbon (FIG. 7G) and oxygen and nitrogen versus activation temperature (FIG. 7H) for the PPy precursor and PPy-T-2 samples.

FIGS. 8A-8B show high pressure (30 bar) CO₂ uptake as a function of N wt % (FIG. 8A) and S wt % (FIG. 8B) in NPC and SPC samples, respectively. Also shown are high pressure CO₂ adsorption uptake for PAn-600-3 compared with PPy-600-2 (FIG. 8C) and SD-M-800-4 compared with PPy-800-4 (FIG. 8D). FIG. 8E shows the dependence of volumetric CO₂ uptake on N content for PPy-T-2 samples in comparison to the PPy precursor measured at different CO₂ pressures. Sorption measurements were performed at 24° C.

FIGS. 9A-9B show high pressure (30 bar) CO₂ uptake as a function of O wt % (FIG. 9A) and Σ(O,N,S) wt % (FIG. 9B) in PC, NPC and SPC samples. Sorption measurements were performed at 24° C.

FIGS. 10A-10B show room temperature volumetric CO₂ (FIG. 10A) and methane (CH₄) (FIG. 10B) adsorption isotherms for PC, NPC, and SPC samples. FIG. 10C shows the molar CO₂:CH₄ uptake ratio as a function of gas pressure for PC, NPC, and SPC samples.

FIGS. 11A-11B show plots of molar CO₂:CH₄ uptake ratio (@ 30 bar) as a function of the surface area (FIG. 11A) and total pore volume (FIG. 11B) for a range of PC, NPC and SPC samples. Sorption measurements were performed at 24° C.

FIGS. 12A-12D show plots of molar CO₂:CH₄ uptake ratio (@ 30 bar) as a function of the surface area (FIG. 12A), total pore volume (FIG. 12B), activation temperature (FIG. 12C), and CO₂ uptake (FIG. 12D) for PPy-T-2 (T=500, 600, 700 and 800° C.) NPC samples. Sorption measurements were performed at 24° C.

FIG. 13 shows high pressure (30 bar) molar CO₂:CH₄ uptake ratio as a function of N wt % in NPC samples. Sorption measurements were performed at 24° C.

FIG. 14 shows the high pressure (30 bar) molar CO₂:CH₄ uptake ratio as a function of C wt % in PC, NPC, and SPC samples. Sorption measurements were performed at 24° C.

FIG. 15 shows volumetric CO₂ uptake of different OPCs activated at increasing temperature, activated carbon and carbon precursor.

FIGS. 16A-16D provide an analysis of the porous structure of OPC samples activated at different temperatures. FIG. 16A shows N₂ adsorption and desorption isotherms for a PC (800) sample. FIG. 16B shows estimated surface area and total pore-volume vs. activation temperature. FIG. 16C shows the distribution of pore volumes as a function of activation temperature as estimated by NLDFT. FIG. 16D shows the surface area (blue bars) and total pore volume (purple) for activated charcoal and eight different PC samples known for high CO₂ uptakes (>14 mmol g⁻¹ at 30 bar).

FIGS. 17A-17B provide additional data relating to the CO₂ uptake of porous carbons. FIG. 17A shows the volumetric CO₂ uptake of various porous carbons prepared from different carbon precursors, including O-rich PC (OPC), N-rich PC (NPC) and S-rich PC (SPC). Measurements were performed in a PCTPRO instrument at 24° C. FIG. 17B shows the graphical representation of surface areas and maximum CO₂ uptake capacities at 30 bar for nine different porous carbon sorbents. The highest CO₂ uptake property (26.6 mmol g⁻¹) is demonstrated by Applicants' newly discovered OPC (750) sample (second bar to the right).

FIGS. 18A-18B provide a demonstration of optimal gas uptake selectivity of OPC samples for CO₂ over CH₄. FIG. 18A shows volumetric CO₂ and CH₄ uptake measurements on OPC (750) sorbents up to a pressure range of 30 bar at 0.5 and 24° C. FIG. 18B shows volumetric CO₂ and CH₄ uptake measurements on commercially available activated charcoal. The molar uptake ratios (CO₂/CH₄) at 30 bar for OPC(750) and activated charcoal are 2.74 and 1.4, respectively.

FIG. 19 provides a demonstration of optimal gas uptake selectivity of OPC samples for CO₂ over CH₄. Volumetric CO₂ and CH₄ uptake measurements on OPC (750) sorbents up to a pressure range of 30 bar at 0.5° C. and 24° C. are shown. The mass uptake ratios (CO₂/CH₄) at 30 bar for OPC(750) are 8.0 and 7.6, respectively.

FIGS. 20A-20B provide a demonstration of the reproducibility of sample preparation and gas uptake properties of OPCs. FIG. 20A shows volumetric CO₂ uptake measurements on four different OPC (750) samples synthesized and activated the same way. FIG. 20B shows two successive CO₂ adsorption and desorption cycles.

FIGS. 21A-21J provide various schemes and data relating to the synthesis and characterization of OPCs. FIG. 21A provides a synthesis scheme for OPC. Photographs of carbon precursor (FIG. 21B), as-synthesized OPC (FIG. 21C), and as-synthesized SPC samples (FIG. 21D) are also shown. OPC samples are pellet like compared to SPC and other PC materials. Scanning electron microscopy (SEM) images of carbon precursor (FIG. 21E), OPC (600) (FIG. 21F) and OPC (800) samples (FIG. 21G) are also shown. FIG. 21H shows an energy-dispersive X-ray spectroscopy (EDS) elemental scan for OPC (800). Also shown are high resolution transmission electron microscopy (TEM) images of OPC (600) (FIG. 21I) and OPC (800) samples (FIG. 21J) showing nm sized micro porous structures.

FIGS. 22A-22B show the isosteric heat of absorption of CO₂ (FIG. 22A) and CH₄ (FIG. 22B) as a function of molar gas uptakes.

FIGS. 23A-23H show the characterization of chemical compositions of carbon precursor and porous carbon samples activated at increasing temperatures. Shown are X-ray photoelectron spectroscopy (XPS) survey scans for C-precursor (FIG. 23A) and OPC (800) (FIG. 23B). Also shown are the wt % of elemental carbon (FIG. 23C) and oxygen (FIG. 23D) vs. activation temperature. XPS elemental scanning for carbon C1s (FIG. 23E) and oxygen O1s (FIG. 23F) are also shown. FIG. 23G shows the Fourier transform infrared spectroscopy (FTIR) spectra of C-precursor and activated OPCs. FIG. 23H shows the Raman spectra and Raman disorder (D) to graphene (G) band intensity ratio vs. activation temperature. The KOH:Polymer weight ratio is 3 in all cases. IR spectra are base line corrected and vertically offset for clarity.

FIGS. 24A-24B provide schemes for the synthesis of various porous carbon materials. FIG. 24A provides a synthetic reaction scheme for the synthesis of furfuryl alcohol-OPC (FFA-OPC) (FeCl₃:FA=10, KOH:PFFA=3, and T=500-800° C.). FIG. 24B provides a synthetic reaction scheme for the synthesis of anisyl alcohol (AA)-OPC (AA-OPC) (KOH:PAA=3, and T=500-800° C.).

FIGS. 25A-25C show photographs of PFFA precursors (FIG. 25A), as-synthesized OPC (FIG. 25B), and as-synthesized SPC samples (FIG. 25C).

FIGS. 26A-26D show SEM images of PFFA (FIG. 26A), precursors and the associated OPCs (FIG. 26B), FFA-OPC₇₅₀ (FIG. 26C), and AA-OPC₇₅₀ (FIG. 26D).

FIGS. 27A-27C show representative high-resolution TEM images of FFA-OPC₆₀₀ (FIG. 27A), FFA-OPC₈₀₀ (FIG. 27B), and AA-OPC₈₀₀ (FIG. 27C) samples showing nanometer sized micro porous structures.

FIGS. 28A-28B show N₂ adsorption isotherms for FFA-OPCs (FIG. 28A) and AA-OPCs (FIG. 28B) measured at liquid N₂ temperature (77K).

FIGS. 29A-29B show estimated apparent surface area (FIG. 29A) and total pore volumes versus activation temperature (FIG. 29B) for FFA-OPC and AA-OPC.

FIG. 30 shows apparent surface area (blue/dark bars) and total pore volumes (purple/light bars) for activated charcoal and PC samples with high CO₂ uptake properties at 30 bar (>12 mmol·g⁻¹).

FIGS. 31A-31B show distribution of pore sizes as a function of pore width for five activation temperatures as determined by the nonlocal DFT method for FFA-OPC (FIG. 31A) and AA-OPC (FIG. 31B).

FIGS. 32A-32D show the wt % of elemental carbon (FIGS. 32A and 32C) and oxygen (FIGS. 32B and 32D) as determined by XPS versus activation temperature for FFA-OPC (FIGS. 32A and 32B) and AA-OPC (FIGS. 32C and 32D).

FIGS. 33A-33B show XPS elemental scanning for oxygen O1s (FIG. 33A) and carbon C1s (FIG. 33B) for PFFA precursor and FFA-OPC.

FIG. 34 shows FTIR spectra of PFFA precursor and activated FFA-OPCs. Spectra are base line corrected and vertically offset for clarity (KOH:PFFA=3 in all cases).

FIGS. 35A-35D show Raman spectra and Raman disorder (D) to graphene (G) band intensity ratio versus activation temperature (KOH:PFFA=3 in all cases).

FIGS. 36A-36B show volumetric CO₂ uptakes of FFA-OPC (FIG. 36A) and AA-OPC (FIG. 36B) samples activated at different temperatures. FIG. 36C shows the volumetric CO₂ uptakes of FAA-OPC₇₅₀, NPC, SPC, and activated charcoal up to a pressure limit of 30 bar. Measurements were performed in a PCTPRO instrument at 24° C.

FIG. 37 shows graphical representation of surface areas and maximum CO₂ uptake capacities at 30 bar for activated charcoal and different sorbent samples with high CO₂ uptake properties at 30 bar (>12 mmol·g⁻¹).

FIGS. 38A-38B show demonstration of reproducibility of sample preparation and gas uptake properties. Volumetric CO₂ uptake measurements are shown on four different batches of FFA-OPC₇₅₀ (FIG. 38A) and AA-OPC₇₅₀ (FIG. 38B) that were synthesized and activated the same way. FIG. 38C shows successive CO₂ adsorption and desorption cycle measurements on an individual FFA-OPC₇₅₀ sample.

FIGS. 39A-39B show a demonstration of ultrahigh CO₂ uptake capability of an FFA-OPC₇₅₀ sample at low temperature. FIG. 39A shows volumetric CO₂ uptake measurements on an FFA-OPC₇₅₀ sample at four different temperatures. At 0.5° C. and 30 bar pressure, sorbent adsorbed an ultrahigh amount of CO₂ that maxed to 43 mmol·g⁻¹ (189 wt %). FIG. 39B shows CO₂ uptakes at four different (labelled) pressures as a function of experiment temperatures.

FIGS. 40A-40B show volumetric CO₂ and CH₄ uptakes measurements on FFA-OPC₇₅₀ (FIG. 40A) and AA-OPC₇₅₀ (FIG. 40B) sorbents up to a pressure range of 30 bar at 0.5 and 24° C. FIG. 40C shows volumetric CO₂ and CH₄ uptake measurements on activated charcoal.

FIGS. 41A-41B show a plot of CH₄ uptake at 30 bar as a function of surface area (FIG. 41A) and total pore volume (FIG. 41B) for FFA-OPC and AA-OPC.

FIGS. 42A-42D show isosteric heat of gas adsorption of CO₂ (FIGS. 42A-B) and CH₄ (FIGS. 42C-D) as a function molar gas uptake for FFA-OPC₇₅₀ (FIGS. 42A and 42C) and AA-OPC₇₅₀ (FIGS. 42B and 42D).

FIGS. 43A-43B show SEM images of the polymer precursor (PTh) (FIG. 43A) and a SPC-2 sample (FIG. 43B).

FIG. 44 shows high pressure volumetric CO₂ uptake as a function of CO₂ pressure on PTh, activated charcoal and activated SPC-700-R samples activated at 700° C. with increasing KOH:PTh weight ratio (r) where r varies from 1 to 5. Experiments were performed at 24° C.

FIG. 45 shows high pressure volumetric CO₂ uptake as a function of CO₂ pressure for two batches of SPC-4 activated at 700° C. Uptake measurements were performed at 24° C.

FIG. 46 shows dependence of CO₂ uptake at the labelled pressure on the KOH:PTh ratio for activated SPC-r samples activated at 700° C. Experiments were performed at 24° C.

FIGS. 47A-47B show dependence of CO₂ uptake at the labelled pressure on surface area (FIG. 47A) and total pore volume (FIG. 47B). SPC samples were synthesized from PTh by activating at 700° C. with different KOH amounts. Experiments were performed at 24° C.

FIGS. 48A-48B show chemical composition of the activated SPC samples by XPS spectroscopy showing the wt % of elemental carbon (FIG. 48A) and oxygen and sulfur (FIG. 48B) versus KOH:PTh ratio.

FIG. 49 shows dependence of CO₂ uptake at the activated SPC samples as a function of carbon composition as determined XPS spectroscopy.

FIG. 50 shows N₂ adsorption isotherms (measured at 77 K) for five different SPC samples activated at 700° C. with KOH:PTh ratios varied from 1 to 5.

FIGS. 51A-51B show estimated surface area (FIG. 51A) and total pore volume (FIG. 51B) versus KOH:PTh ratio for five different SPC samples activated at 700° C.

FIG. 52 shows pore size distributions for the samples in FIGS. 51A-51B.

FIG. 53 shows a high resolution transmission microscope (HRTEM) image of a SPC-2 sample. Scale bar=10 nm.

FIGS. 54A-54B show the total pore volume, volume of macropores (>50 nm), mesopores (>2 nm), micropores (<2 nm), and narrower micropores (<1 nm) as a function of KOH:PTH ratio (FIG. 54A), and percentages of total pore volumes for micropores, narrower micropores and mesopores versus KOH:PTH ratio (FIG. 54B).

FIGS. 55A-55C show percentages of pore volumes for micropores, narrower micropores and mesopores versus CO₂ uptake.

FIG. 56 shows high pressure volumetric CH₄ uptake as a function of CH₄ pressure on activated SPC-700-R samples activated at 700° C. with increasing KOH:PTh ratio (r) where r varies from 1 to 5. Experiments were performed at 24° C.

FIG. 57 shows dependence of CH₄ uptake at labeled pressure on KOH:PTh ratios for activated SPC samples. Experiments were performed at 24° C.

FIGS. 58A-58D show various data related to SPC samples. FIG. 58A shows the molar CO₂:CH₄ uptake ratio as a function of a gas pressure for SPC sorbents activated with different KOH:PTh ratio. Also shown are plots of molar CO₂:CH₄ uptake ratios at 30 bar as a function of KOH:PTh ratio (FIG. 58B), surface area (FIG. 58C), and total pore volume (FIG. 58D). Experiments were performed at 24° C.

DETAILED DESCRIPTION

It is to be understood that both the foregoing general description and the following detailed description are illustrative and explanatory, and are not restrictive of the subject matter, as claimed. In this application, the use of the singular includes the plural, the word “a” or “an” means “at least one”, and the use of “or” means “and/or”, unless specifically stated otherwise. Furthermore, the use of the term “including”, as well as other forms, such as “includes” and “included”, is not limiting. Also, terms such as “element” or “component” encompass both elements or components comprising one unit and elements or components that comprise more than one unit unless specifically stated otherwise.

The section headings used herein are for organizational purposes and are not to be construed as limiting the subject matter described. All documents, or portions of documents, cited in this application, including, but not limited to, patents, patent applications, articles, books, and treatises, are hereby expressly incorporated herein by reference in their entirety for any purpose. In the event that one or more of the incorporated literature and similar materials defines a term in a manner that contradicts the definition of that term in this application, this application controls.

There are generally two classes of materials employed for carbon dioxide (CO₂) separation: reactants and adsorbents. The former includes amine and other reactive species such as ionic liquids and alkali-metal-based oxides. At present, monoethanolamine (MEA) is the industry standard. However, regeneration, degradation and corrosion, together with health and environmental issues, still affect its large scale implementation.

Impregnation of CO₂ capture materials onto supports has been investigated, but it is only recently that the regeneration temperature has been lowered by their combination with carbon nanomaterials. Ionic liquids, suitable for high pressure capture, are expensive and toxic, while cheap alkali metal oxides suffer from severe deactivation upon cycling.

Although the aforementioned materials show optimal selectivity between CO₂ and methane (CH₄), their myriad drawbacks have meant that much effort has been invested into the study of solid porous sorbents, such as porous carbons (PC), metal-organic frameworks (MOFs), microporous zeolites, and porous silica-based sorbents with high surface area.

MOFs outperform zeolites in terms of maximum capacity at high pressure, but are expensive since they require complex multistep synthesis procedures. In addition, their gas adsorption capacity degrades after several cycles of usage. Carbonaceous materials, such as activated carbon and charcoal, are cheaper and less sensitive to moisture than zeolites and MOFs, but their adsorption capacity generally increases with loss of selectivity at high pressure.

Chemically activated porous carbon adsorbents have large surface areas and pore volumes associated with micro- and meso-porous structure. As a result, such materials show significantly improved CO₂ capturing capacity as compared to traditional carbonaceous materials.

It has been suggested that the presence of nitrogen or sulfur dopants is responsible for improved CO₂ uptake in porous carbon materials (e.g., Nat Commun., 2014, 5, 3961 and U.S. Pat. Pub. No. 2015/0111024). These studies were undertaken at 30 bar (1 bar=100,000, Pa=750.06 mmHg) using compounds previously reported to show improved results over activated carbon at 1 bar (e.g., Adv. Funct. Mater., 2011, 21, 2781-2787; and Microporous Mesoporous Mater., 2012, 158, 318-323). The improved high pressure results were proposed to be due to the S or N centers acting as a Lewis base to facilitate the ambient polymerization of the CO₂. However, previous investigations of the role of N-doping in CO₂ capture by PCs up to 1 bar pressure shows no correlation (e.g., ACS Appl. Mater. Interfaces, 2013, 5, 6360-6368).

The conventional goal in synthesizing a porous carbon material with optimal CO₂ adsorption is to focus on increased surface area and pore volume (e.g., U.S. Pat. Pub. No. 2016/0136613). The same approach is presumed to also work for the separation of CO₂ from natural gas.

However, the present disclosure demonstrates that increasing the surface area and pore volume of a carbon material do not guarantee the best adsorbent. Instead, a combination of factors is involved in defining the ideal porous carbon absorbent material.

In some embodiments, the present disclosure pertains to novel materials for CO₂ capture. In additional embodiments, the present disclosure pertains to methods of making the materials of the present disclosure. In further embodiments, the present disclosure pertains to methods of utilizing the materials of the present disclosure for the capture of CO₂ from various environments. As set forth in more detail herein, the present disclosure can have various embodiments.

Materials for CO₂ Capture

In some embodiments, the present disclosure pertains to materials for CO₂ adsorption at pressures above 1 bar. In some embodiments, the materials include a porous material with a surface area of at least 2,800 m²/g, and a total pore volume of at least 1.35 cm³/g. In some embodiments, a majority of pores of the porous material have diameters of less than 2 nm as measured from N₂ sorption isotherms using the BET (Brunauer-Emmett-Teller) method.

In some embodiments, more than about 50% of pores of the porous material have diameters of less than 2 nm. In some embodiments, more than about 60% of pores of the porous material have diameters of less than 2 nm. In some embodiments, more than about 70% of pores of the porous material have diameters of less than 2 nm. In some embodiments, more than about 80% of pores of the porous material have diameters of less than 2 nm. In some embodiments, between about 50% to about 90% of pores of the porous material have diameters of less than 2 nm.

In some embodiments, the present disclosure pertains to materials for the separation of CO₂ from natural gas at partial pressures of either component above 1 bar. In some embodiments, the materials include a porous material with a surface area of at least 2,200 m²/g, and a total pore volume of at least 1.00 cm³/g. In some embodiments, a majority of pores of the porous material have diameters of greater than 1 nm and less than 2 nm as measured from N₂ sorption isotherms using the BET (Brunauer-Emmett-Teller) method.

In some embodiments, more than about 50% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm. In some embodiments, more than about 60% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm. In some embodiments, more than about 70% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm. In some embodiments, more than about 80% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm. In some embodiments, between about 50% to about 90% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm.

In some embodiments, the porous materials of the present disclosure include a porous carbon material. In some embodiments, the porous carbon material has a carbon content of between 80% and 95% as measured by X-ray photoelectron spectroscopy. In some embodiments, the porous materials include a porous carbon material with a surface area of at least 2,800 m²/g, a total pore volume of at least 1.35 cm³/g, and a carbon content of between 80% and 95% as measured by X-ray photoelectron spectroscopy. In some embodiments, the materials include a porous carbon material with a surface area of at least 2,000 m²/g, a total pore volume of at least 1.00 cm³/g, and a carbon content of greater than 90% as measured by X-ray photoelectron spectroscopy.

The porous carbon materials of the present disclosure may be prepared in various manners. For instance, in some embodiments, the porous carbon material is prepared by heating an organic polymer precursor or biological material in the presence of potassium hydroxide (KOH). In some embodiments, the temperature of activation is between 700° C. and 800° C. In some embodiments, the temperature of activation is between 600° C. and 700° C.

The materials of the present disclosure can have various chemical components. For instance, in some embodiments, the materials of the present disclosure are rich in oxygen. As such, in some embodiments, the materials of the present disclosure are referred to as oxygen rich activated porous carbons (OPCs). In some embodiments, the materials of the present disclosure have an oxygen content of more than about 10 wt %. In some embodiments, the materials of the present disclosure have an oxygen content between about 10 wt % and about 25 wt %.

In some embodiments, the materials of the present disclosure may lack other heteroatoms, such as nitrogen or sulfur. For instance, in some embodiments, the total heteroatom content of the materials of the present disclosure may range from about 0 wt % to about 1 wt %. In some embodiments, the total heteroatom content of the materials of the present disclosure may be less than about 1 wt %.

The materials of the present disclosure can have various advantageous properties. For instance, in some embodiments, the materials of the present disclosure have high surface areas. In some embodiments, the materials of the present disclosure have surface areas of more than about 1,000 m²/g. In some embodiments, the materials of the present disclosure have surface areas that range from about 1,000 m²/g to about 5000 m²/g (e.g., Table 5). In some embodiments, the materials of the present disclosure have surface areas of about 3005 m²/g (e.g., in OPC samples chemically activated at 800° C.) (e.g., FIG. 16D).

In some embodiments, the materials of the present disclosure have high CO₂ adsorption capacities. In some embodiments, the materials of the present disclosure have a CO₂ adsorption capacity of more than about 100 wt %. In some embodiments, the materials of the present disclosure have CO₂ adsorption capacities between about 117 wt % and about 189 wt %.

In some embodiments, the materials of the present disclosure have a CO₂ adsorption capacity of up to 117 wt % (26.6 mmol/g) at a pressure of 30 bar, a number that is higher than any reported uptake values for activated porous carbon (PC) adsorbents (e.g., FIGS. 17A-17B and Table 5). In some embodiments, the materials of the present disclosure capture CO₂ from a natural gas containing environment that is rich in CH₄ at a maximum molar uptake ratio of 2.75 (7.5 by mass ratio) at a pressure of 30 bar (e.g., FIGS. 18A-18B and FIG. 19).

In some embodiments, the materials of the present disclosure (e.g., OPCs that are activated at 750° C., referred to herein as OPC (750)) outperform most of the existing porous carbons for high pressure uptake of CO₂ (e.g., 26.6 mmol/g; 117 wt % at 30 bar) and demonstrate optimal selectivity for CO₂ capture over CH₄ uptake (e.g., V_(CO2)/V_(CH4) ratio ˜2.7 (molar) and ˜7.5 (by wt) at 30 bar) at room temperature. Additionally, OPC (750) demonstrates ultrahigh CO₂ uptake (43 mmol g⁻¹; 189 wt %) at 0.5° C., a value that was never reported previously (e.g., FIG. 18A).

In some embodiments, the materials of the present disclosure exhibit remarkable thermal stability and reproducible gas uptake properties for many cycles (e.g., FIGS. 20A-20B). Unlike other fine powder type activated porous carbon materials, the materials of the present disclosure can be clumpy and pelletized in some embodiments. Such properties can in turn make the materials of the present disclosure better candidates for preparing solid pellet-like adsorbents (e.g., FIG. 21C).

Formation of Materials

The materials of the present disclosure can be prepared in various manners. Additional embodiments of the present disclosure pertain to methods of making the materials of the present disclosure.

In some embodiments, a carbon precursor is first synthesized. Next, the carbon precursor is activated to form porous carbon materials. Various methods may be utilized to optimize sample preparation to synthesize activated porous carbon materials with very high CO₂ uptake.

In some embodiments, a carbon precursor is activated by chemical activation. In some embodiments, the chemical activation includes heating the carbon precursor in a mixture. In some embodiments, the carbon precursor is heated in a mixture that contains a base, such as KOH. In some embodiments, the heating temperature ranges from about 500° C. to about 800° C. (FIG. 15). In some embodiments, the activation temperature is about 750° C.

In some embodiments, the carbon precursor is synthesized by polymerizing a carbon source. In some embodiments, the polymerization occurs by exposing the carbon source to an oxidant, such as iron (III) chloride (FeCl₃) in the presence of acetonitrile (CH₃CN).

In some embodiments, the materials of the present disclosure are prepared from affordable and readily available carbon sources. In some embodiments, the carbon sources include oxygen-containing carbons. In some embodiments, the oxygen containing carbon sources are rich in alcohol. In some embodiments, the carbon sources lack heteroatoms such as nitrogen, sulfur, and combinations thereof. As such, in some embodiments, the formed materials of the present disclosure also lack such heteroatoms.

In some embodiments, the materials of the present disclosure are prepared by heating a biological material. In some embodiments, the biological material includes, without limitation, sawdust, coconut husk, and combinations thereof. In some embodiments, the biological material is chosen from at least one of the following: sawdust and coconut husk.

In some embodiments, the carbon source that is utilized to make the materials of the present disclosure is furfuryl alcohol (FFA) (e.g., purchasable from Sigma Aldrich at a price of $354 for 25 kg with purity >98%) (e.g., Table 4). In some embodiments where the carbon source is FFA, the formed carbon precursor is polyfurfuryl alcohol (PFFA).

In some embodiments, the materials of the present disclosure are prepared by heating an organic polymer precursor or biological material. In some embodiments, the organic polymer precursor or biological material includes oxygen in a functional group. In some embodiments, the functional group is a furyl. In some embodiments, the organic polymer precursor is FFA. In some embodiments, the organic polymer precursor polymerizes to form polyfurfuryl alcohol (PFFA). In some embodiments, PFFA is prepared by the polymerization of furfuryl alcohol with a catalyst. In some embodiments, the catalyst is FeCl₃.

In some embodiments, the functional group is an anisyl. In some embodiments, the organic polymer precursor polymerizes to form polyanisyl alcohol (PAA). In some embodiments, PAA is prepared by the polymerization of anisyl alcohol with a catalyst. In some embodiments, the catalyst is a protic acid.

A more specific method of making the materials of the present disclosure is illustrated in FIG. 21A. In this illustration, the FFA is polymerized by using FeCl₃ as the oxidant. In a typical synthesis, a solution of FeCl₃ is prepared by solubilizing FeCl₃ in CH₃CN. FFA is then mixed with CH₃CN and slowly added to the FeCl₃ solution. The mixture is then magnetically stirred for 24 hours at room temperature. The polymerized product, brown colored PFFA, is then separated by filtration over a sintered glass funnel, washed thoroughly with abundant distilled water, and then with acetone. This is followed by drying at 40° C. for 12 hours. The yield of the final product was ˜98%.

Next, the porous carbon was chemically activated by heating a PFFA-KOH mixture (KOH/PFFA at a weight ratio of 3) in inert atmosphere. The mixture was then placed inside a quartz tube/tube furnace setup and heated for 1 hour at a fixed temperature in the 500-800° C. range, under a flow of Ar. The activated OPC sample was then thoroughly washed several times with diluted HCl and distilled water and dried on a hot plate at 70° C. for 12 hours.

In some embodiments, the KOH/PFFA ratio can be varied. In some embodiments, the activation temperatures and the PFFA-KOH mixing procedure can be varied.

Use of Materials for Gas Capture

The materials of the present disclosure can be utilized to capture and selectively remove various gases (e.g., CO₂, CH₄, and combinations thereof) from various environments. Additional embodiments of the present disclosure pertain to methods of utilizing the materials of the present disclosure for the separation of a mixture of gases by preferential adsorption and selective desorption. Further embodiments of the present disclosure pertain to methods of utilizing the materials of the present disclosure for the capture of CO₂ from various environments. In some embodiments, the environments include an atmosphere or an environment that contains a mixture of gases. In some embodiments, the methods of the present disclosure pertain to processes for separating CO₂ from natural gas by exposing the natural gas to the materials of the present disclosure.

In some embodiments, the methods of the present disclosure utilize the materials of the present disclosure in a process in which selectivity and separation of two gases (such as CH₄ and CO₂) is accomplished by a combination of an adsorption process that favors one of the components (e.g., selectivity of CO₂ over CH₄). Thereafter, the desorption of the two components from the carbon materials can be significantly different by control over various parameters, such as temperature, pressure, and combinations thereof. In some embodiments, such control allows for the specific desorption of one of the components prior to the other (e.g., CH₄ over CO₂). In some embodiments, the overall process allows for the selective separation of at least two gaseous components.

In some embodiments, the materials of the present disclosure differentiate between CH₄ and CO₂ adsorption as well as desorption. In some embodiments, the selectivity of adsorption is further enhanced since the pressure/temperature dependencies of the desorption of CH₄ and the desorption of CO₂ are distinct from each other such that they may be used to improve separation. Thus, in some embodiments, a mixture of adsorbed CH₄ and CO₂ will desorb under different conditions: the CH₄ first and the CO₂ second. In some embodiments, this difference means that the overall adsorption/desorption selectivity of CH₄ and CO₂ is higher than prior materials.

In some embodiments, the materials of the present disclosure can be used for the selective capture of CO₂ from various environments. In some embodiments, the materials of the present disclosure can be utilized for the selective capture of CO₂ over hydrocarbons in the environment (e.g., CH₄). In some embodiments, the adsorption of CO₂/CH₄ mixtures and measurement of the desorption selectivity can be varied.

Applications and Advantages

The methods and materials of the present disclosure can provide numerous advantages. For instance, in some embodiments, the methods and materials of the present disclosure can be utilized for the selective removal of CO₂ from natural gas (e.g., methane) that contains various amounts of CO₂ (e.g., 10-20 mol % of CO₂). Such an application is an important goal in the field of oil and natural gas, since contaminant CO₂ decreases its power efficiency. For an ideal gas adsorbing material, the major requirements are as follows: it should be cheap, simple to synthesize, demonstrate reproducible and high gas uptake property, and complete desorption of CO₂ at low pressure. In various embodiments, the materials of the present disclosure possess all of these properties.

In some embodiments, the methods and materials of the present disclosure can be utilized for the separation of CO₂ from natural gas at a source where low to medium levels of CO₂ are present. In some embodiments, the methods and materials of the present disclosure can be used as a secondary recovery method for treating CH₄/CO₂ mixtures in which CO₂ is the major component. In some embodiments, such mixtures include high-pressure samples that are the result of an initial CH₄/CO₂ separation using traditional methods.

The materials of the present disclosure can also provide numerous advantages. In particular, among the most efficient solid sorbents for capturing CO₂ from natural gas or atmosphere, MOFs and KOH aided chemically activated PC materials with large surface areas and micro pores have been investigated for decades. PC composites demonstrate remarkable thermal stability and repeatability for selective gas uptake measurements.

However, to date, most of the researchers have synthesized porous carbons from carbon rich precursors that contain heteroatoms, such as nitrogen or sulfur. For sulfur rich precursors, the most common feedstock for synthesizing PCs are polythiophene or poly(2-thiophenemethanol), whereas, pyrrole of acrylonitrile are being utilized for the production of nitrogen containing PCs.

Unfortunately, the high cost of both chemicals hinders the industrial scale use of PCs produced from these materials. Based upon an analysis of the best PC materials in terms of selectivity and CO₂ uptake, Applicants have noted that the common link is not the presence of strong Lewis base species such as N or S, but the presence of oxygen. Thus, Applicants envision that oxygen is an important component for selectivity and high adsorption of gases (e.g., CO₂ and/or CH₄) in the materials of the present disclosure.

Additional Embodiments

Reference will now be made to more specific embodiments of the present disclosure and experimental results that provide support for such embodiments. However, Applicants note that the disclosure below is for illustrative purposes only and is not intended to limit the scope of the claimed subject matter in any way.

Example 1. Preparation of Porous Carbon Materials

This Example provides processes for the preparation of various porous carbon materials.

Example 1.1. Synthesis of Activated Porous Carbon (PC) from Coconut Shell

Pieces of dry coconut shell were placed inside a quartz tube/tube furnace setup and carbonized for 1 hour at 450° C., under a flow of Ar (flow rate 500 sccm). The carbonized product (500 mg) was thoroughly mixed with potassium hydroxide (KOH) powder (1.0 g). The mixture was then placed inside a quartz tube/tube furnace setup, dried for 20 minutes and then heated for 1 hour at a fixed temperature of 600° C. under continuous flow of Argon (flow rate of about 600 sccm), washed with distilled water (ca. 4 L) and then with acetone (ca. 1 L) and dried at 80° C. for 12 hours.

Example 1.2. Synthesis of Nitrogen-Containing Porous Carbon (NPC) from Polypyrrole

The polymerized carbon precursor polypyrrole was synthesized using FeCl₃ as a catalyst following a modification of Applicants' previous methods. In a typical synthesis, a solution of FeCl₃ (50 g) in CH₃CN (200 mL) was prepared. Next, a solution of pyrrole (5.0 g) in CH₃CN (50 mL) was slowly added to the previous solution. The mixture was stirred for 24 hours. The polymerized product was then separated by filtration, washed thoroughly with distilled water (ca. 4 L) and then with acetone (ca. 1 L) and dried at 80° C. for 12 hours. The yield of the final product was ˜98%. The polypyrrole was chemically activated by heating with an excess (2 or 4 fold by weight) of KOH in inert atmosphere. In a typical activation process, polypyrrole (500 mg) was thoroughly mixed with KOH (1.0 g) that had been crushed to a fine powder in a mortar. The mixture was then placed inside a quartz tube within a tube furnace, dried for 20 minutes and then heated for 1 hour at a fixed temperature in the 500-800° C. range, under a flow of Ar (flow rate 600 sccm). The activated samples were then thoroughly washed with diluted HCl (1.4 M, 100 mL) and several times with distilled water until the filtrate attained neutral pH 7. Finally, the activated PC was dried on a hot plate at 70° C. for 12 hours.

Example 1.3. Synthesis of Polyfurfuryl Alcohol (PFFA)

In a typical synthesis, a solution was prepared by dissolving FeCl₃ (50 g) in CH₃CN (200 mL). To this a solution of furfuryl alcohol (5 g, Sigma Aldrich, 98%) mixed with CH₃CN (50 mL) was slowly added. The mixture was stirred for 24 hours under continuous argon purging. The polymerized product, brown colored polyfurfuryl alcohol (PFFA) was separated by filtration, washed thoroughly with DI water (ca. 4 L) and acetone (500 mL), before being dried at 40° C. for 12 hours under vacuum (Yield=98%).

Example 1.4. Conversion of PFFA to Oxygenated Porous Carbon (OPC)

In a typical activation process, PFFA (500 mg) was thoroughly mixed with KOH powder (1.5 g, crushed previously) in a mortar for 10 minutes. The mixture was then placed inside a quartz tube/tube furnace, dried for 20 minutes and then heated for 1 hour at 500, 600, 700 or 750° C., under a flow of Ar (99.9%, flow rate 600 sccm). The activated samples were then washed with HCl (100 mL, 1.4 M) and DI water until the filtrate attained pH=7. The product was dried at 70° C. for 12 hours under vacuum. The yield of activated PC materials depended on the activation temperature: OPC₅₀₀=55%, OPC₆₀₀=40%, OPC₇₀₀=30%, and OPC₇₅₀=25-27%.

Example 2. Characterization of the Porous Carbon Materials

This example provides various data relating to the characterization of the porous carbon materials in Example 1.

The volumetric uptake measurements (sorption and desorption) of CO₂ and CH₄ by the porous carbons were performed in an automated Sievert instrument (Setaram PCTPRO). Various PC samples were first crushed into powders and packed in a stainless steel autoclave sample cell. Initial sample pre-treatment was carried out at 130° C. for 1.5 hours under high vacuum. The free volume inside the sample cell was determined by a series of calibration procedures done under helium. Gas uptake experiments were carried out with high purity research grade CO₂ (99.99%) and CH₄ (99.9%) at 24° C.

FIG. 1A shows a plot of the uptake of CO₂ at 30 bar as a function of the apparent Brunauer-Emmett-Teller (BET) surface area (S_(BET)) for all the PC adsorbent measured. As expected, an increase in surface area correlates with an increase in CO₂ uptake. However, any value above 2,800 m²g⁻¹ does not appear to improve adsorption. Thus, continued attempts to create even higher surface area materials may not result in any further improvements in CO₂ uptake.

It is envisioned that increased total pore volume (V_(p)) will facilitate increased CO₂ adsorption. However, as shown in FIG. 1B, it appears that for pore volumes over 1.35 cm³g⁻¹, there is not a resulting greater uptake.

The aforementioned trends were for the highest pressures. However, the homologous series PPy-T-2 (T=500, 600, 700, and 800° C.) along with the precursor (PPy) allows for a comparison across a range of pressures.

FIGS. 1C and 1D show the relationship between CO₂ uptake and BET surface area (FIG. 1C) and total pore volume (FIG. 1D) for different pressures in the range of 5-30 bar. As expected, these plots clearly show a significant effect of pressure on the CO₂ uptake (i.e., higher pressures result in higher uptake). Moreover, the point at which increased surface area (or total pore volume) does not increase CO₂ uptake decreases with decreased pressure. Thus, whereas at 30 bar CO₂ pressure increasing the surface area above 2,800 m²g⁻¹ does not improve adsorption, at 5 bar this value decreases to 1300 m²g⁻¹ (FIG. 1C). This suggests a greater diminution of returns in attempting to create high surface area adsorbents if lower pressures are to be used in the system. The effect is similar for total pore volume, where at 5 bar it appears that any pore volume over 0.5 cm³g⁻¹ does not result in greater uptake.

Furthermore, a linear trend has been observed between surface area and pore volume for the majority of the samples studied (FIG. 1E). However, many of the samples show a divergence from the trend. The aforementioned results demonstrate a higher pore volume than expected. Furthermore, Applicants note that the aforementioned porous carbons have some of the highest CO₂ uptake performances.

FIG. 2 shows the relationship between the activation temperature and the CO₂ uptake for the porous carbon samples listed in Table 1. The general trend is increasing uptake with increased activation temperature with a possible maximum between 700 and 800° C.

TABLE 1 Summary of PC, NPC, and SPC samples studied with their elemental analysis, physical properties and CO₂ uptake. Surface Total pore CO₂ uptake at 30 bar C O N S area volume and 24° C. Sample^(a) (wt %)^(b) (wt %)^(b) (wt %)^(b) (wt %)^(b) S_(BET) (m² g⁻¹) (cm³ g⁻¹)^(c) (mmol · g⁻¹) Activated 94.10 5.90 0.00 0.00 845 0.43 8.45 charcoal BPL^(d) 91.3 8.7 0.00 0.00 951 0.49 8.66 SD-600-4 82.24 15.80 0.00 0.00 2290 1.10 20.52 SD-800-4 89.96 8.03 0.00 0.00 2850 1.35 22.90 CN-600-2 88.13 11.87 0.00 0.00 1250 0.64 13.50 PPy-500-2 72.47 17.19 10.33 0.00 1255 0.53 12.60 PPy-600-2 74.78 19.72 5.49 0.00 2013 1.03 18.98 PPy-700-2 90.01 9.87 0.14 0.00 2956 1.45 22.98 PPy-800-2 91.39 8.60 0.00 0.00 3230 1.51 21.01 PPy-800-4 90.78 9.11 0.10 0.00 3450 2.57 22.10 PAn-600-3 84.50 6.75 8.75 0.00 1410 1.38 14.50 SD-M-800-4 85.39 8.15 6.46 0.00 2990 2.69 23.80 PTh-600-2 64.91 25.88 0.00 9.21 2256 1.02 18.81 PTh-700-2 82.47 13.01 0.00 4.51 1980 0.99 20.32 PTh-800-2 88.18 7.24 0.00 4.58 2890 1.43 22.87 ^(a)Precursor-temperature-KOH:precursor ratio. ^(b)Determined by XPS. ^(c)Determined at P/P_(o) ~0.99. ^(d)Purchased from Calgon Carbon Corp.

Given the relationships between surface area and pore volume with CO₂ uptake, it is not surprising that their relationship with activation temperature is also similar (FIGS. 3A-3B). The analysis of a series of samples prepared from N-containing polymer polypyrrole (PPy) at different activation temperatures (i.e., PPy-T-2), but otherwise under identical conditions, allows for a convenient direct comparison of the effects of temperature.

The CO₂ uptake plot for each sample as a function of CO₂ pressures is shown in FIG. 4, whereas FIG. 5 shows their corresponding N₂ adsorption isotherms at 77 K. It may be noticed that the shape of these isotherms is dependent on the activation temperature. The isotherm for PPy-800-2 is much steeper than that of PPy-500-2 between relative pressures of 0.4 and 1.0, indicating the variation in mesoporosity and adsorption capacity. For the homologous series of NPC materials, the estimated surface area (S_(BET)) and the total pore volume (V_(p)) gradually increase with activation temperature (FIGS. 6A-6B), describing the incremental trend for mildly to strongly activated samples. Between 500 and 700° C., the surface area and total pore volume increases systematically, whereas for temperatures above 700° C. no significant increment is noticed.

Besides the surface area and pore volume, another important characteristic that can be obtained from the N₂ adsorption isotherms is the pore size distribution (PSD) of the porous solid. FIGS. 7A-7H depict the PSDs for three different PPy-based PCs prepared under mild (T=500° C.) to strong (T=800° C.) activation conditions. The distribution plot for T=500° C. indicates that the activated PC mainly consists of micropores in the 1-2 nm range, whereas the plot for PPy-700-2 clearly shows signature of some larger pores in the 2-3.5 nm range. The most strongly activated PC and PPy-800-2 even shows a significant number of mesopores in the 3-6 nm range, in agreement with the steeper adsorption registered for relative pressures of more than 0.4.

A comparison of the variation in pore size and distribution (FIG. 7A) with the CO₂ uptake for the same samples (FIG. 4 and Table 1) was also made. From 500° C. to 700° C., there is a dramatic increase in the high pressure uptake, which can be associated with the generation of pores in the range of 2-3 nm. However, as may be seen from FIG. 4, there is a slight (but significant) decrease upon further activation to 800° C., even though there is an increase in the presence of larger pores. This suggests that larger pores are not necessarily ideal for a high CO₂ adsorption. The pore size distribution for the other top adsorbents studied shows a similar bi-modal pore structure centered on 1 nm and 1.5-2 nm.

The structural and textural morphology of the activated PPy-T-2 samples were characterized by scanning electron microscopy (SEM). FIG. 7B shows that the activated NPC contains multiple layers projected vertically upward and surfaces that are full of micron sized holes. In order to image the microporous structure of the activated sample, high resolution transmission electron microscopy (HRTEM) was utilized.

FIG. 7C displays an image demonstrating randomly distributed micropores with dimensions in the range of 0.5-1 nm for a PPy-600-2 sample. These and the images of the other samples are in agreement with the BET measurements.

Given the hazardous nature of working with KOH, the amount used in the activation process is of importance with regard to any scalability issues. Applicants have recently shown that KOH provides greater activation than borates. However, based upon the present data set for PPy-800-n (n=2, 4), it is clear that increasing the KOH:precursor ratio from 2 to 4 does not result in a change in the CO₂ uptake profile (FIG. 7D), despite a dramatic (70%) increase in the pore volume (Table 1).

Applicants note that PPy-800-4 has one of the highest surface areas (3450 m²g⁻¹) measured for any PC sorbent. However, PPy-800-4 is less efficient than PPy-800-4 between 10-20 bar.

The CO₂ uptake for NPC and SPC samples as a function of their N or S content is shown in FIGS. 8A-8B. For both NPC (FIG. 8A) and SPC (FIG. 8B) samples, the CO₂ uptake is at a maximum with the heteroatom content of less than 5 wt %. The chemical composition of polypyrrole precursor and activated PPy derived NPC samples were determined by XPS (Table 1). The identity and wt % of the elements present on the sample surface were determined by XPS survey scans (e.g., FIGS. 7E and 7F). These spectra revealed that the precursor polypyrrole and activated NPCs are primarily composed of C, O, and N. It should be noted that the O content of NPCs has been observed, but discounted as significant, except as a potential source as both Lewis acid and base moieties. Applicants note that H content is not provided by XPS data, and so percentage values measured by other techniques will vary.

As a result of chemical activation and the activation temperature, the wt % of all elements changes (Table 1). The general trend is that the wt % of C increases, whereas that of O and N decreases gradually with increasing activation temperature. The compositional dependence on the activation temperature is demonstrated for the PPy-T-2 samples (FIG. 7G). The first point to note is that the N content decreases consistently with activation temperature (FIG. 7H). However, there is a distinct step in the O composition between 600 and 700° C. (FIG. 7H), which is mirrored in the C wt % composition (FIG. 7G). However, it is important to note that while at the highest activation temperatures the N content becomes negligible, the O content remains significant.

An equally interesting variation was observed for SPC samples (Table 1). The C content stays essentially constant between the PTh precursor and the product activated at 600° C., despite the S composition decreasing. The reason for this anomaly is the oxidation of the PC material as measured by the increased O content. As with the N content in the NPC samples, the S composition in the SPC samples decreases to a low value at the highest activation temperatures.

Based upon these results, it would appear that the presence of neither N nor S correlates in a positive manner with the CO₂ uptake, although in the present case a higher heteroatom content is associated to lower surface area and pore volume, hence the corresponding lower CO₂ uptake. Nonetheless, the limited effect of the presence of heteroatoms on CO₂ uptake is in line with previous results, and Applicants' proposal that the presence of N or S is not responsible for any stabilization of poly-CO₂ that has been proposed to be responsible for high CO₂ adsorption at 30 bar.

Furthermore, the source of the heteroatom also appears to affect the physical parameters and hence the CO₂ uptake. For example, the use of polyacrylonitrile (I, PAn) instead of polypyrrole (II, PPy) makes a significant difference suggesting the chemical speciation of the N content is important (FIG. 8C). In addition, the use of a poly-N containing heterocycle, melamine, as the N source results in an improvement in the performance (FIG. 8D). However, it is unclear whether this is a cause or effect. If the amount of CO₂ adsorbed is divided by the total pore volume one gets a similar value for both for PPy-800-4 and SD-M-800-4. Thus, the CO₂ uptake is determined by the total pore volume, but the pore volume is clearly a function of the precursor, rather than the process conditions.

As was noted with the pressure dependence of the CO₂ uptake on the surface area and total pore volume, the uptake appears to be less affected by the N content at lower pressures. Thus, as shown in FIG. 8E, the greatest CO₂ uptake at 30 bar for NPC requires N <2 wt %. However, if measured at 5 bar the uptake is almost independent of N content at values <10 wt %. This again suggests that the need to create specialty adsorbents diminishes with decreased operating pressure.

Both NPC and SPC samples contain significant O, as do the PC samples produced from non-heteroatom containing precursors. Given that some of the PC samples perform in a comparable manner to those of NPC or SPC, N and S composition cannot be the sole key to high adsorption. While the presence of more than 5 wt % of either N or S appears to significantly lower the uptake of CO₂, although this could be related to the lower surface area of the heteroatom-rich samples, the O content is far more effective for the high CO₂ adsorption observed with 3-16 wt % O (FIG. 9A).

In support of this observation, there are also some significant findings on the CO₂ capture capacity of activated PCs obtained from the carbonization of asphalt with KOH. The reduction with H₂ of asphalt-derived N-doped PCs causes a significant increase of capture capacity up to 26 mmol·g⁻¹. The XPS elemental analysis of the sample before and after H₂ treatment shows that the sample with higher CO₂ capacity undergoes a significant increase of O content while the N content and type is only slightly changed. This finding supports Applicants' hypothesis that O plays a major role in establishing the CO₂ capture capacity of PCs. However, what appears to be more important is the combined presence of a heteroatom (i.e., Σ(O,N,S), FIG. 9B). This can be alternatively stated that the C content should be between 80-95 wt %.

Based upon the forgoing, it is possible to identify the parameters that define a PC material for maximum CO₂ uptake: have a surface area ≥2,800 m²g⁻¹, a pore volume ≥1.35 cm³g⁻¹, and a C content between 80-95 wt %. To achieve these performance parameters it is necessary to activate above 700° C. and to ensure full mixing of the KOH with the precursor. It is significant that the first two of these suggest that developing higher and higher surface area materials is unproductive, and that understanding the third may lead to the design of new PC materials. Furthermore, these values offer additional variance when the uptake of CO₂ is required at lower pressures.

Applicants have also investigated the CO₂/CH₄ selectivity by measuring CO₂ and CH₄ uptake isotherms up to a high pressure limit of 10, 20 and 30 bar at 24° C. A summary of the data is shown in Table 2.

TABLE 2 Summary of PC, NPC, and SPC samples studied with their molar gas uptakes and selectivity for CO₂ over CH₄ at different uptake pressures. Molar CO₂ uptake CH₄ uptake (CO₂:CH₄) uptake (mmol · g⁻¹) at (mmol · g⁻¹) at ratio Sample^(a) 10 bar 20 bar 30 bar 10 bar 20 bar 30 bar 10 bar 20 bar 30 bar Activated 6.27 7.51 8.45 4.28 5.44 6.03 1.46 1.38 1.41 charcoal BPL 6.30 7.87 8.66 3.24 4.96 6.18 1.94 1.59 1.40 SD-600-4 12.06 16.77 20.52 5.23 7.54 8.52 2.31 2.22 2.41 SD-800-4 13.61 18.78 22.90 6.65 9.45 10.92 2.05 1.99 2.10 CN-600-2 10.91 12.65 13.50 5.94 7.24 7.96 1.83 1.74 1.70 PPy-500-2 9.51 11.27 12.60 4.11 5.06 5.98 2.31 2.23 2.11 PPy-600-2 11.37 16.45 18.98 5.39 6.33 7.41 2.11 2.60 2.56 PPy-700-2 12.50 18.12 22.98 5.75 7.92 9.41 2.17 2.29 2.44 PPy-800-2 11.94 17.21 21.01 5.78 8.23 9.82 2.07 2.09 2.14 PPy-800-4 11.18 16.51 22.11 5.10 7.33 8.83 2.19 2.25 2.50 PAn-600-3 8.19 10.84 14.50 4.04 5.26 6.03 2.03 2.06 2.40 SD-M-800-4 12.09 18.70 23.76 5.58 8.12 9.41 2.17 2.30 2.52 PTh-600-2 11.17 15.42 18.81 4.77 6.12 7.37 2.34 2.52 2.55 PTh-700-2 11.51 16.67 20.32 4.62 6.87 8.01 2.49 2.43 2.54 PTh-800-2 13.10 18.80 22.87 5.81 8.55 10.14 2.25 2.20 2.26 ^(a)Precursor-temperature-KOH:precursor ratio.

FIG. 10A shows the CO₂ uptake plots along with the corresponding CH₄ uptake results in FIG. 10B. Additionally, the molar uptake selectivity (CO₂/CH₄) is defined by the molar ratio of adsorbed CO₂ and CH₄ at a certain pressure, i.e., at 30 bar. The dependence of molar uptake selectivity for a sorbent as a function of corresponding gas pressure is depicted in FIG. 10C. It is significant that for any particular sample, the selectivity varies with gas pressure. Of the samples investigated, PPy-600-2 demonstrated highest selectivity of 2.56 at 30 bar.

FIG. 11A shows a plot of molar CO₂:CH₄ uptake ratio as a function of the surface area (S_(BET)) for all the PC adsorbents measured. For low surface area samples, there is an increase in selectivity with increased surface area. However, as with uptake, further increase in surface area above 2000 m²g⁻¹ does not appear to improve selectivity. In a similar manner, increased total pore volume (V_(p)) does facilitate increased selectivity, but only to a pore volume of 1.00 cm³g⁻¹. No improvement in performance is shown above the aforementioned value (FIG. 11B).

The series PPy-T-2 (T=500-800° C.) allows for the direct comparison of homologous materials. In this case, it appears that the values of 2,000 m²g⁻¹ and 1.00 cm³g⁻¹ for the surface area and total pore volume (FIGS. 12A-12D) represent maxima rather than thresholds. It is possible that for any homologous series similar maxima are observed. However, the thresholds observed in FIGS. 11A-11B are useful indicators.

From Table 2, it can be seen that an activation temperature of 600° C. is a minimum for good selectivity. However, from FIG. 12C, it may be seen that for the series PPy-T-2 (T=500-800° C.), this value is actually an optimum. Such results may vary with a particular class of material. However, a lower activation temperature is required to create a material with good selectivity as compared to optimum CO₂ uptake (FIG. 12D), suggesting that the best attainable sorbent material will have to combine a wise trade off of selectivity and CO₂ capture capacity. As may be seen from a comparison of PPy-800-2 and PPy-800-4 (Table 2), increased KOH concentration during the activation step results in greater selectivity.

The molar CO₂:CH₄ uptake ratio for NPC samples as a function of their N content is shown in FIG. 13. The selectivity for measurements at 30 bar decreases with N content above 5 wt %. In the case of SPC, there appears to be no effect on selectivity with S content (Tables 1 and 2).

These results seem to suggest that the presence of neither N nor S correlates in a direct manner with the CO₂/CH₄ selectivity. This is in line with Applicants' previous proposal. However, in this Example, a higher heteroatom content implies a lower surface area (and total pore volume) of the sorbent materials. Hence, a definite lack of impact of N or S doping on the selectivity performance of PCs cannot be considered a priori. Significantly, as may be seen from the data in Table 2, at lower pressures (10 bar), there is almost no dependence between selectivity and heteroatom content.

As was observed with the uptake efficiency for CO₂, the selectivity appears to be more a function of the total heteroatom composition (i.e., Σ(O,N,S) wt %, as presented in FIG. 14 in terms of C wt % (=100−Σ(O,N,S) wt %)). However, based upon the analysis of all the PC, NPC, and SPC materials studied, the 0 wt % seems to be the major contributor. The CO₂/CH₄ selectivity is at a potential maximum as long as C content is below 90 wt % (i.e., for Σ(O,N,S)>10 wt %). At lower pressure (10 bar), the carbon content is possibly even higher (C<94 wt %).

A study of a wide range of PC, NPC, and SPC materials under high pressure CO₂ and CH₄ adsorption offers some useful insight into the parameters that may collectively control both the CO₂ uptake efficiency and the CO₂/CH₄ selectivity. A summary of the proposed key requirements for a PC material with either good CO₂ uptake or good CO₂/CH₄ selectivity is given in Table 3 based on the results presented herein.

TABLE 3 Summary of proposed parameters required for optimum CO₂ uptake and CO₂/CH₄ selectivity for PC, NPC, and SPC. Parameter Uptake @ 30 bar Selectivity @ 30 bar Surface area (m²g⁻¹) >2800 >2000 Total pore volume (cm³g⁻¹) >1.35 >1.0 Temperature of activation (° C.) 700-800 600 Carbon content (%) 80-95 <90

As far as CO₂ uptake is concerned, any porous carbon material with a surface area of more than 2,800 m²g⁻¹ at 30 bar is unlikely to be improved (when prepared from the KOH activation of non-nanostructured precursors). A similar threshold appears to be true for the total pore volume of the material (1.35 cm³g⁻¹). This suggests that seeking synthetic routes to ever higher surface area and/or high pore volume PC-based adsorbents is counterproductive.

However, it should be understood that if uptake at lower pressures is desired, these threshold values decrease even further. This result is highly important in considering the choice of adsorbent to be used in a large scale unit. The adsorbent intended for use in a low pressure system needs a lower surface area and pore volume to perform than a potentially more expensive to manufacture material. It also impacts the formation of pelletized materials for adsorbent bed applications, since the formation of the pellet through inclusion of a binder inevitably lowers the surface area and pore volume. Applicants' results suggest that for lower pressure applications, this is not important since the uptake is less dependent on extremely high surface areas and/or pore volumes.

Given the prior interest in N- and S-doped PC materials, the results show that CO₂ uptake is inversely related to S and N content in SPC and NPC, respectively. However, due to the preparation process used in this Example (KOH activation), there is an intrinsic dependence between heteroatom content and surface area (total pore volume) in all sorbents. In particular, higher surface areas imply lower N or S contents.

Consequently, the use of KOH activated PCs in industrial scale units must take into account that a higher heteroatom content cannot offset the corresponding drop of CO₂ capture performance due to a decrease of surface area of the materials. In practical terms, it is the Σ(O,N,S) wt % or C wt % (=100−Σ(O,N,S) wt %) that is the defining factor for CO₂ uptake. This is true irrespective of the source of the heteroatom. However, O appears to be the main factor, since a C content of between 80 and 95 wt % offers the potential for high CO₂ uptake. However, at these levels, if the make-up is N or S, the uptake is likely reduced. It should also be observed based upon the source of the heteroatom that if heteroatoms are to be incorporated and “active”, they are preferentially included using heterocycle precursors, such as melamine in the case of N, rather than other heteroatom-rich structures.

It may be assumed that the parameters that makes a good CO₂ adsorbent may be the same as those that make a selective material. However, Applicants' results indicate that the two are only broadly related. The levels of surface area and pore volume can be even lower for good CO₂/CH₄ selectivity, as compared to CO₂ uptake (Table 3).

In summary, Applicants demonstrate in this Example that a synthetic goal for PC-based material, for both high CO₂ adsorption and high CO₂/CH₄ selectivity, would comprise a C content of less than 90%. Given that neither N nor S seem to have a significant effect rather than the O that is present, it is clear that a design C_(x)O_(1-x) where x<0.9 would possibly make an ideal CO₂ adsorbent material with the best CO₂/CH₄ selectivity. Furthermore, the goal should be a precursor where oxygen is incorporated into a cyclic moiety.

Additional experimental results and information are provided in FIGS. 15-23H and Tables 4-6. For instance, the chemical composition of OPC (750) has been thoroughly characterized via XPS, FTIR and Raman spectroscopy (FIGS. 23A-23H and Table 6), while textural properties were determined by high resolution scanning electron microscopy (FIGS. 21E-H), transmission electron microscopy (FIGS. 21I-J) and a BET Surface area analyzer (FIGS. 16A-16D). Moreover, measured values for gas uptakes have been confirmed via volumetric, gravimetric, multiple sample and cycles experiments.

To the best of Applicants' knowledge, oxygen-rich carbon materials prepared from furfuryl alcohol has never been investigated for high pressure uptake of CO₂ and CH₄. In fact, there have been no reports of its use as a precursor for oxygen-rich porous carbon materials. In addition, a higher value for the isosteric heat of adsorption of CO₂ (23 kJ·mol⁻¹) versus 13 kJ·mol⁻¹ for CH₄ allows Applicants to scheme a temperature dependent strategy for removing CO₂ from natural gas via selective adsorption and desorption of CH₄ and CO₂ in steps (FIGS. 22A-22B).

TABLE 4 Survey of different c-feedstock used for the synthesis of various PCs with high CO₂ uptake properties. Maximum CO₂ uptake at 30 PC Source material for SKU pack size bar (mmol sample C-precursor CAS no. (Sigma Aldrich) Price g⁻¹) (wt %) SPC (1) 2- 636-72-6 181315-100 G $155 for 100 g 18.4 (81) Thiophenemethanol SPC (2) Thiophene 110-02-1 T31801-500 G $47 for 500 g — NPC (1) Pyrrole 109-97-7 W338605-1 KG $315 for 1 kg — NPC (2) Polyacrylonitrile 25014-41-9 181315-100 G $190 for 100 g 16.8 (74) OPC (l) Furfuryl alcohol 98-00-0 W249106-25 KG $60 for 1 kg  26.6 (117) ($354 for 25 kg) OPC (2) Furan 110-00-9 185922-500 ML $54 for 500 mL —

TABLE 5 Survey of gas adsorption properties of various PCs with high CO₂ uptake capacity. Ratio of Uptake of CO₂ Uptake of CO₂ Uptake of CH₄ absorbed Surface at 30 bar at 10 bar at 30 bar CO₂/CH₄ area S_(BET) (mmol · g⁻¹) (mmol · g⁻¹) (mmol · g⁻¹) at 30 bar Sample (m²g⁻¹) (wt %) (wt %) (wt %) (molar) (mass) C-Precursor 48 3.3 (14.5) 1.6 (7.0) — — OPC (500) 1143 17.1 (75.2) 8.6 (37.8) 6.7 (10.7) 2.5 (7.0) OPC (600) 2116 20.0 (88.1) 12.5 (55.0) 8.3 (13.3) 2.3 (6.3) OPC (700) 2610 20.8 (91.5) 12.7 (55.9) 9.1 (14.6) 2.3 (6.2) OPC (750) 2856 26.6 (117.0) 15.1 (66.4) 9.6 (15.5) 2.75 (7.5) OPC (750) 2856 42.9 (188.9) 18.5 (81.5) 14.6 (23.4) 2.93 (8.0) at 0.5° C. OPC (800) 3005 23.0 (101.2) 12.9 (56.7) 9.01 (14.4) 2.5 (7.0) SPC ^(a) 2500 18.4 (81.0) 10.0 (44.0) 7.1 (11.3) 2.6 (7.1) r-NPC ^(a) 1450 16.8 (74.0) 7.1 (31.2) 7.6 (12.2) 2.2 (6.1) Act. charcoal 845 8.4 (36.9) 6.3 (27.7) 6.0 (9.6) 1.4 (3.8)

TABLE 6 Elemental composition of various types porous carbon materials as determined by XPS excluding the contribution from elemental H. C O Surface Total pore (wt %) (wt %) KOH:pre- area S_(BET) volume V_(P) Sample XPS XPS cursor (m² g⁻¹) (cm³g⁻¹) C-Precursor 69.91 30.09 — 48 0.02 OPC (500) 77.49 22.51 3:1 1143 0.78 OPC (600) 82.04 17.74 3:1 2216 1.19 OPC (700) 85.07 14.93 3:1 2610 1.46 OPC (750) 88.21 11.79 3:1 2856 1.77 OPC (800) 89.28 10.72 3:1 3005 1.92 Act. charcoal 94.10 5.90 3:1 845 0.43

Example 3. Optimizing Carbon Dioxide Uptake and Carbon Dioxide-Methane Selectivity of Oxygen-Doped Porous Carbon Prepared from Oxygen Containing Polymer Precursors

In this Example, Applicants report a reproducible synthesis of oxygen containing PC (OPC) by KOH activation at 500-800° C. of two oxygen containing precursor polymers: polyfurfuryl alcohol (PFFA) and polyanisyl alcohol (PAA), yielding FFA-OPC and AA-OPC, respectively. Both OPCs exhibit remarkable thermal stability and reproducible gas uptake properties for multiple cycles. Unlike other fine powder type activated PC materials, as-synthesized OPC sorbents are large pellet-like, making them a better candidate for preparing binder free solid pellet-like sorbent. The surface area and pore volumes of the OPC are independent of the precursor identity, but controlled by the activation temperature.

Similarly, the uptake of CO₂ is determined by the physical properties of the OPC: activation at 750° C. results in uptake that equals or out-performs existing PCs for high pressure uptake (30 bar) at 24.0° C. (FFA-OPC₇₅₀: 117 wt %; AA-OPC₇₅₀: 115 wt %). In contrast, while the uptake of CH₄ for both OPCs is greatest for samples activation at 750° C., FFA-OPC₇₅₀ shows significantly greater uptake compared to AA-OPC₇₅₀, 15.5 wt % versus 13.7 wt %, respectively. As a consequence, AA-OPC₇₅₀ demonstrates optimal selectivity for CO₂ capture over CH₄ uptake (AA-OPC₇₅₀: V_(mass)(CO₂/CH₄)=8.37 at 30 bar) as compared to other reported PCs. A higher value for the isosteric heat of adsorption of CO₂ (33 kJ mol⁻¹) versus CH₄ (14 kJ mol⁻¹) suggests a new temperature dependent strategy for removing CO₂ from natural gas via selective adsorption and desorption cycles. The differences in performance for CH₄ uptake between FA-OPC and AA-OPC, prepared under identical conditions, suggests that the structure of the precursor (heterocyclic versus exocyclic oxygen) is an additional variable in the design of new OPC materials.

In this Example, Applicants envision that an ideal PC would have a surface area of more than 2,800 m²g⁻¹, a pore volume of more than 1.35 cm³g⁻¹, and an oxygen content of between 5-20%. In this regard, Applicants have concentrated research efforts in developing routes to such materials that allow for low cost and reproducible synthesis.

Furfuryl alcohol (FA) has previously been formed into a highly cross-linked precursor via acid catalysis that can be converted to a PC. However, the process results in only a modest surface area and adsorption not sufficient to reach the performance parameters listed above. In this Example, Applicants report that, through FeCl₃ catalyzed polymerization and activation of polyfurfuryl alcohol (PFFA), an oxygenated PC (OPC) sorbent may be prepared which demonstrated higher room temperature CO₂ uptake as compared to other PC materials.

Applicants have shown that process conditions (temperature and KOH:precursor ratio) control the formation of micro (<2 nm) versus meso (>2 nm) porosity that is responsible for the highest CO₂ uptake. Although there appeared to be no significant difference in the performance as a function of the precursor, in creating nitrogen-doped PCs (NPCs) it was noted that incorporation of nitrogen into 6 versus a 5-membered cyclic precursor made a significant difference in the performance. Thus, there is interest as to whether using identical process conditions the precursor makes any significant effect. Applicants have therefore investigated a new OPC precursor polyanisyl alcohol (PAA) to compare with PFFA.

Moreover, if OPC is to be scaled, the cost of the catalyst used for PFFA, and similar polymeric precursors, is an issue to be addressed. Anisyl alcohol (4-methoxybenzyl alcohol), which is used as fragrance and flavourant and thus produced on a large scale, represents a low cost OPC precursor, while the formation of the polymer feedstock for OPC, polyanisyl alcohol (PAA), is synthesized by treating anisyl alcohol with concentrate H₂SO₄ in a single step.

Example 3.1. Materials and Methods

FeCl₃, furfuryl alcohol (Sigma Aldrich, 98% purity), anisyl alcohol (Sigma Aldrich, 98% purity), CH₃CN, powdered KOH, distilled water, acetone, HCl, Ar (99.9% pure), CO₂ (99.99% pure, Matheson TRIGAS) and CH₄ (99.9% pure) were used as supplied. The SPC and NPC samples used as comparison were synthesized from 2-thiophenemethanol and polyacrylonitrile (Sigma Aldrich), respectively, following protocols previously described.

The chemical composition of the polymer and porous carbon materials were determined by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The XPS measurements were carried out in a PHI Quantera scanning XPS microprobe. The wt % of chemical elements was determined by XPS survey scans with pass energy of 140 eV.

For detailed elemental analysis high-resolution multi-cycle elemental scans with pass energy 26 eV was performed. Each spectrum was then deconvoluted by appropriate basis functions. Before spectral fitting, each spectrum was corrected for reference binding energy for C1s to 284.8 eV. FTIR spectral measurements were performed in a Nicolet FTIR Infrared Microscope equipped with a liquid N₂ cooled detector. Raman spectra of solid samples were measured in a Renishaw Raman microscope equipped with a 514 nm excitation laser. Scanning electron microscopic images were obtained by a FEI Quanta 400 ESEM FEG high-resolution field emission scanning electron microscope. The high-resolution TEM images of activated OPCs were obtained by a JEOL 2100 field emission gun transmission electron microscope.

The textural properties: surface areas, distributions of pore volumes and total pore volume of carbonaceous materials were obtained by analyzing N₂ sorption isotherms (measured at 77 K), measured in a Quantachrome Autossorb-3b BET Surface Analyzer.

The surface area (S_(BET)) was calculated by the multipoint BET (Brunauer-Emmett-Teller) method. Before measurements, samples were dried at 140° C. for 12 hours under high vacuum system equipped with a liquid N₂ cold trap. The apparent BET surface area (S_(BET)) of the activated PC samples was calculated from the N₂ adsorption isotherm in the partial pressure (P/P₀) range of 0.05-0.30 and the total pore volume (V_(P)) was estimated from the amount of N₂ adsorbed at P/P₀=0.99. The distributions of pore volumes were determined by analyzing the data via non-local density functional theory. Selected results are given in Table 7.

Example 3.2. Synthesis of Polyfurfuryl Alcohol (PFFA)

In a typical synthesis, a solution was prepared by dissolving FeCl₃ (50 g) in CH₃CN (200 mL). To this, a solution of furfuryl alcohol (5 g, Sigma Aldrich, 98%) mixed with CH₃CN 50 (mL) was slowly added. The mixture was stirred for 24 hours under continuous argon purging. The polymerized product, brown colored polyfurfuryl alcohol (PFFA) was separated by filtration, washed thoroughly with DI water (ca. 4 L) and acetone (500 mL), before being dried at 40° C. for 12 hours under vacuum (Yield=98%).

Example 3.3. Synthesis of Polyanisyl Alcohol (PAA)

In a typical synthesis, concentrated H₂SO₄ (˜6 mL) was slowly added dropwise to a glass beaker containing anisyl alcohol (10 g) in three steps. In each step, H₂SO₄ (2 mL) was added in drops to the glass beaker, stirred with a glass stirrer and a purple colored solid polymer of polyanisyl alcohol was formed. The synthesized polymer was separated from the mixture. To avoid over heating of reactants, the glass beaker was kept surrounded by a water/ice mixture. The reaction process continued until all the anisyl alcohol was converted into solid polymer. The synthesized polymer was washed with DI water (4×50 mL) to remove excess acid and then with acetone (200 mL). The solid polymer was then crushed into powder, transferred to a glass beaker and quickly washed with acetone (100 mL) and dried at room temperature for 12 hours under vacuum. The final product was a dark brown colored semi-soft mixture of PAA and trace amount of acetone, which helps mixing of polymer with KOH before chemical activation.

Example 3.4. Conversion of Polymer Precursors to Oxygenated Porous Carbon (OPC)

In a typical activation process, either PFFA or PAA (500 mg) was thoroughly mixed with KOH powder (1.5 g, crushed previously) in a mortar for 10 minutes. The mixture was then placed inside a quartz tube/tube furnace, dried for 20 minutes and then heated for 1 hour (for AA-OPC 30 minutes) at 500, 600, 700, 800 or 750° C., under a flow of Ar (99.9%, flow rate 600 sccm). The activated samples were then washed with HCl (100 mL, 1.4 M) and DI water until the filtrate attained pH=7. The product was dried at 70° C. for 12 hours under vacuum. The yield of activated PC materials depended on the activation temperature (e.g., FFA-OPC₅₀₀=55%, FFA-OPC₆₀₀=40%, FFA-OPC₇₀₀=30%, FFA-OPC₇₅₀=25-27%, and FFA-OPC₈₀₀=15%).

Example 3.5. CO₂ and CH₄ Uptake Measurements

The volumetric uptake measurements (pressure dependent excess isotherms) of CO₂ and CH₄ were performed in an automated Sievert instrument (Setaram PCTPRO). Various OPC samples were first crushed into powders and packed in a stainless steel autoclave sample cell. Initial sample pre-treatment was carried out at 130° C. for 1.5 hours under high vacuum. The free volume inside the sample cell was determined by a series of calibration procedures done under helium. Gas uptake experiments were carried out with high purity research grade CO₂ (99.99% purity, Matheson TRIGAS) and CH₄ (99.9% purity). The gravimetric uptake measurements were performed in a Rubotherm magnetic suspension balance instrument (Rubotherm, Germany). A summary of selected results is given in Table 7.

TABLE 7 CO₂ and CH₄ uptake properties (@ 24° C.) in comparison with commercial PC samples of OPC samples prepared from polyfurfuryl alcohol (FFA-OPC) and polyanisyl alcohol (AA-OPC). Total Surface pore Molar Mass area volume Uptake CO₂ Uptake CO₂ Uptake CH₄ ratio ratio S_(BET) V_(p) @ 10 bar @30 bar @30 bar CO₂/CH₄ CO₂/CH₄ Sample^(a) (m²g⁻¹)^(e) (cm³g⁻¹)^(f) (mmol · g⁻¹) (wt %) (mmol · g⁻¹) (wt %) (mmol · g⁻¹) (wt %) @30 bar @30 bar FFA-OPC₅₀₀ 1143 0.78 8.6 37.8 17.1 75.2 6.7 10.7 2.5 7.0 FFA-OPC₆₀₀ 2116 1.19 12.5 55.0 20.0 88.1 8.3 13.3 2.4 6.6 FFA-OPC₇₀₀ 2610 1.46 12.7 55.9 20.8 91.5 9.1 14.6 2.3 6.3 FFA-OPC₇₅₀ ^(b) 2856 1.77 15.1 66.4 26.6 117.0 9.6 15.5 2.8 7.6 (18.5) (81.5) (42.9) (188.9) (14.6) (23.4) (2.9) (8.0) FFA-OPC₈₀₀ 3005 1.92 12.9 56.7 23.0 101.2 9.0 14.4 2.5 7.0 AA-OPC₈₀₀ 853 0.49 6.7 29.7 9.5 41.6 3.8 6.1 2.7 7.3 AA-OPC₆₀₀ 1980 1.13 11.6 50.9 17.6 77.3 6.8 10.9 2.6 7.1 AA-OPC₇₀₀ 2700 1.54 11.9 52.6 22.4 98.5 7.9 12.7 2.8 7.8 AA-OPC₇₅₀ ^(b) 3310 1.87 13.9 61.0 26.0 114.5 8.5 13.7 3.0 8.4 (17.6) (77.5) (39.3) (172.9) (10.5) (16.8) (3.7) (10.3) AA-OPC₈₀₀ 3040 2.27 9.6 42.2 21.8 96.0 8.3 13.2 2.6 7.2 Act. Charcoal^(c) 845 0.47 6.3 27.6 8.5 37.2 6.0 9.7 1.4 3.9 BPL^(d) 951 0.53 6.30 27.7 8.7 38.1 6.18 9.9 1.4 3.8 ^(a)OPC activation temperature. ^(b)Values in parenthesis performed at 0.5° C. ^(c)Purchased from Mallinckrodt chemical works. ^(d)Purchased from Calgon carbon corp. ^(e)Apparent BET surface area estimated in P/P₀ range of 0.05-0.30. ^(f)Total pore volume measured at P/P₀ ~0.99

Example 3.6. Results and Discussion

The experimental procedures for the synthesis of the best PC sample using KOH activation with the highest CO₂ uptake property rely on the optimization of two major parameters: the KOH:PFFA ratio and the temperature of activation (T_(a)). Earlier reports suggest that the overall porosity and the surface area of a chemically activated PC material increase with KOH concentration, and initial results suggested that KOH:PFFA=3 is best for developing nano-sized micropores (1-2 nm range). Thus, Applicants' procedures were carried out by keeping KOH:PFFA ratio fixed to 3 and by activating premixed KOH-PFFA mixtures at a single temperature in the range of 500-800° C.

A schematic outlining the synthetic protocol for OPC from FA is presented in FIG. 24A. In the first step, a solid powder-like polymer of FA was prepared from liquid FA by the reaction in the presence of a FeCl₃ catalyst in CH₃CN. The synthesis of polyanisyl alcohol (PAA) is more facile and involves treating anisyl alcohol with concentrate H₂SO₄ in a single step (FIG. 24B).

The PFFA and PAA were then chemically activated by mixing with pre-ground KOH followed by pyrolysis at a stable temperature in the 500-800° C. range. A significant advantage of the use of the PFFA prepared in this manner, as compared to previous methods, is that it only releases a small amount of volatile product during activation, unlike other PC precursors.

FIGS. 25A-25B show representative images of the PFFA and the resulting FFA-OPC₇₅₀. Both the as prepared OPC has a large pellet-like morphology. In comparison, an image of sulfur containing porous carbon (SPC) sample prepared under identical activation conditions is shown in FIG. 25C. This important structural rigidity makes OPC sorbents more appropriate for practical applications as opposed to other powder like sorbents.

The structural and textural morphology of the as-synthesized polymer and resulting activated OPC samples were determined by scanning electron microscopy (SEM) as represented by FIGS. 26A-26D. The PFFA and PAA precursors have a relatively dense morphology, but after activation with KOH, the resulting OPCs exhibit a texture full of micron size holes, multiple corners and edges that are absent in the precursors.

The related energy dispersive X-ray spectroscopy (EDS) determined elemental composition confirms the OPCs are primarily composed of carbon and oxygen (Table 8).

TABLE 8 Elemental composition of OPC as determined by XPS and EDS. XPS EDS Sample C (wt %) O (wt %) C (wt %) O (wt %) PFFA-precursor 69.91 30.09 68.64 31.36 FFA-OPC₅₀₀ 77.49 22.51 78.64 18.85 FFA-OPC₆₀₀ 82.04 17.74 87.09 12.91 FFA-OPC₇₀₀ 85.07 14.93 89.92 10.08 FFA-OPC₇₅₀ 88.21 11.79 90.12 8.08 FFA-OPC₈₀₀ 89.28 10.72 90.58 9.42 PAA-precursor^(b) 74.90 21.41 AA-OPC₅₀₀ 76.66 23.34 AA-OPC₆₀₀ 83.36 13.64 AA-OPC₇₀₀ 89.37 10.63 AA-OPC₇₅₀ 91.01 8.99 AA-OPC₈₀₀ 91.27 8.73 ^(a) Contributions from elemental H were excluded. ^(b)PAA-precursor contained 3.7% S residue from the acid catalyst.

In order to image the microporous structure of activated OPCs, high-resolution transmission electron microscopy (HRTEM) was utilized. FIGS. 27A-27C display a set of images demonstrating randomly distributed micropores with dimension in the range of 1-2 nm for FFA-OPC₆₀₀ and slightly larger but evenly distributed micropores for FFA-OPC₈₀₀ and AA-OPC₈₀₀ samples. These nano-sized micropores play key roles in the ultra high CO₂ uptake at higher pressure.

Further characterization to determine important structural parameters such as surface area, pore size distribution and the total pore volumes of C-precursor and different OPC specimens activated at a fixed temperature in the range of 500-800° C. with a fixed KOH/PFFA ratio of 3 was carried out by measurement of the N₂ adsorption isotherms (at 77 K) using a BET (Brunauer-Emmett-Teller) surface area analyzer.

FIGS. 28A-28B show such set of isotherms for the OPCs activated at labelled temperature. Difference in the shape of these isotherms was noticed depending on the activation temperature. The isotherm for FFA-OPC₈₀₀ was much steeper than that of FFA-OPC₅₀₀ up to a relative pressure of 0.4 (FIG. 28A), indicating the variation in microporosity and adsorption capacity. The isotherm for AA-OPC₈₀₀ was shallower than that of the other samples, suggesting meso pore generation.

The estimated surface area (S_(BET)) and the total pore volume (V_(p)) gradually increased with activation temperature (FIGS. 29A-29B) describing the incremental trend for mildly to strongly activated samples. Activation between 650° C. and 800° C. the surface area for FFA-OPCs varied smoothly than that for pore volumes (FIG. 29A). In contrast, the surface area for AA-OPCs reaches an apparent maximum at 750° C. (FIG. 29B). Irrespective of these differences, both parameters increase with increasing activation temperature, with activation above 750° C. giving materials with surface area and pore volume above the threshold (i.e., >2,800 m²g⁻¹ and >1.35 cm³g⁻¹, respectively) to enable maximum CO₂ uptake.

FIG. 30 provides a comparison with other reported carbon based activated sorbents such as activated charcoal, SPC, NPC and asphalt derived PC specifically, explored for high pressure CO₂ uptakes. The values for AA-OPC₇₅₀ are amongst of the highest reported surface area and pore volume values for carbon based PC samples reported to date.

FIGS. 31A-31B depict the distributions of pore sizes as a function of pore width for activation temperatures (500° C.≤T_(a)≤800° C.) to strong (T_(a)=800° C.) activation conditions. These plots show that samples activated at temperatures between 500 and 700° C. mainly consisted of micropores in the range of 1-2 nm. In contrast, the distribution plot for FFA-OPC₈₀₀ indicates that chemical activation at temperature of about 800° C. created some additional mesopores in the 2.0-3.5 nm range, confirming the findings from HRTEM images discussed earlier (FIG. 27B). Pores larger than 4 nm were practically absent in all samples, except for AA-OPC₈₀₀ where the mesopore (>2 nm) contribution is dominant (FIG. 31B).

As noted above, the best CO₂ uptake of a PC is observed with a carbon content of 80-95 wt %. The chemical composition of polymer precursors and the subsequent OPCs was determined by X-ray photoelectron spectroscopy (XPS). The identity and wt % of the elements present on the sample surface were determined by XPS survey scans for core level electrons (Table 8). The XPS data further confirms that OPC samples primarily contained C and O (the H contents are not shown in XPS).

As expected, the C and O content varied from the polymers to the OPCs during chemical activation, and for the OPC samples the general trend was that the wt % of C increased and O decreased gradually with increasing activation temperature (FIGS. 32A-32D). Based upon the analysis, samples activated between 600° C. and 800° C. fall within the range required for maximum CO₂ uptake.

A set of high resolution XPS elemental scan data for C1s and O1s of PFFA and FFA-OPC samples (FIGS. 33A-33B), de-convoluted by appropriate basis peaks helped Applicants identify the possible functional groups present in the precursor and numerous activated samples. For PFFA, the C1s band could be resolved into four main peaks and labelled according to probable functional groups as in polythiophene. Thus, these peaks were assigned to the following functional groups: sp² hybridized C═C (284.7 eV), C—C (286.1 eV), C—O—C (287.1 eV) and C═O (288.9 eV). An additional shoulder near 291.3 eV is attributed to π-π* shake-up peak. In contrast to PFFA, the activated sample exhibited much narrower C═C peak (FWHM: 1.3 eV versus 2.2 eV for PFFA). The resolved basis peaks under O1s spectra were attributable to two main functional groups: the C—O—C group at 533.2 eV (O within the furan ring) and the carbonyl group (C═O) at 531.8 eV. Without being bound by theory, Applicants believe that KOH induced oxidation during chemical activation at higher temperature resulted in formation of more carbonyl groups, though the absence of well resolved band for O1s made it difficult to extract the absolute proportion of these two functionalities.

The nature of carbon and oxygen functional groups present in the as-synthesized PFFA and activated FFA-OPCs were further explored via FTIR spectroscopy. The IR spectrum for PFFA (FIG. 34) exhibited well defined but broad peaks originated from various IR active vibrational stretches identified as: C—O—C asymmetric stretching vibration (1020 cm⁻¹), C—C stretching (1358 cm⁻¹), and C═C symmetric and asymmetric stretching vibrations in the furan ring (1510 and 1585 cm⁻¹, respectively). The frequency shift and broadness of these bands may be attributed to the aggregated phase of the synthesized polymer. In contrast to PFFA, the activated samples exhibited multiple stretching vibrations with gradually decreasing intensity. The intensity of all these peaks decreased systematically with increasing activation temperature as evidenced by the spectra of FFA-OPC₅₀₀ and FFA-OPC₈₀₀. For FFA-OPC₅₀₀, in addition to C═C stretching frequencies another shoulder peak was identified near 1710 cm⁻¹, which could be assigned to C═O stretching vibrations. Moreover, the C—O—C and O—H functional groups that were present in the C-precursor and mildly activated samples (T_(a)=500° C.) slowly removed due to chemical activation at higher temperature.

The aromatic sp² hybridized C═C and amorphous sp³ hybridized C—C bonds present in PFFA and activated FFA-OPCs were further characterized by Raman spectroscopy. FIGS. 35A-35C represent a set of three normalized spectra depicting spectral changes for two major bands; the sharp graphene (G) band (1590 cm⁻¹) and the broad disorder (D) band located (1360 cm⁻¹), attributing to the aromatic sp² and amorphous sp³ hybridized carbons, respectively. A more direct dependence for the (D/G) intensity ratio on activation temperature is shown in FIG. 35D, bottom panel. The D/G value varied from 0.56 for PFFA, to 0.75 for FFA-OPC₅₀₀ and 0.83 for FFA-OPC₈₀₀, signifying the gradual removal of sp² and addition of sp³ carbons as a result of chemical activation under mild to strong activation conditions.

Example 3.7. CO₂ Uptake

Among the various types of solid porous materials that efficiently capture CO₂, MOFs (metal organic frameworks), zeolites, cross-linked polyethylenimine, and a variety of powder like activated PCs play significant roles over other sorbents and widely utilized for industrial application due to their high surface area, thermal stability, low cost of synthesis, and high gas adsorption capacity with remarkable reproducibility. However, within the category of activated PC materials with high CO₂ uptake capacity, to date, most of the sorbents were investigated for CO₂ uptake performance up to a pressure limit of only 1 bar due to the limitation of available instruments. However, in industrial applications, higher pressures are needed.

For example, removal of CO₂ from natural gas at the wellhead needs to be optimized between 15-30 bar. Applicants have previously shown that the best PC materials show a maximum CO₂ uptake of 20-25 mmol·g⁻¹ at 30 bar and 24° C. In this context, Applicants have carried out careful volumetric CO₂ uptake experiments on the OPC sorbents as a function of gas pressure up to a limit of 30 bar and compared the results to previously reported PC samples measured under the same conditions.

In order to identify the OPC sorbent with the highest CO₂ uptake capacity, Applicants measured pressure dependent CO₂ uptake for a set of OPC samples prepared with a fixed KOH:polymer=3 and activated at a fixed temperature in the 500-800° C. range (FIGS. 36A-36C). Clear difference between the shapes of uptake isotherms indicates that uptake varies with activation temperature (T_(a)). In particular, higher values of T_(a) correlated with higher uptake values for a specific adsorption pressure.

The C-precursors adsorbed negligible amount of CO₂ and the most mildly activated OPC. AA-OPC₅₀₀ showed an uptake similar to activated charcoal. The general trend was: OPC that was activated at higher temperature contained more micropores and with larger surface area performed better. Strikingly, both FFA-OPC₇₅₀ and AA-OPC₇₅₀ captured more CO₂ than their OPC₈₀₀ homologs; while in the case of FFA-OPC₈₀₀ it has a higher surface area and pore volume (Table 7).

A similar observation was reported for low pressure (up to 1 bar) gas uptake of CO₂ by polypyrrole derived PCs. Applicants believe that the reason for the higher uptake performance of OPC₇₅₀ was that OPC₈₀₀ contained more mesopores larger than 2 nm. In this context, it is important to note that up to a pressure bar of 5 bar all OPC samples capture similar amount of CO₂ (except the OPC₅₀₀ samples).

FIG. 36C displays a set of equilibrium volumetric CO₂ uptakes at room temperature as a function of adsorbate pressure for various PC specimens such as activated charcoal, NPC, SPC, FFA-OPC₇₅₀, and AA-OPC₇₅₀.

Applicants noticed that among the all activated OPCs, though all of them captured more CO₂ than previous reported SPC or NPC samples, both FFA-OPC₇₅₀ and AA-OPC₇₅₀, demonstrated the highest CO₂ capture capability. Thus, Applicants subsequently carried out repeated uptake experiments on different batches of both OPC₇₅₀. The CO₂ uptake result for FFA-OPC₇₅₀ was further verified by another gravimetric uptake experiment carried out in a Rubotherm magnetic suspension balance instrument (FIG. 36C, open circles). These uptake plots further established that at a pressure of 30 bar, FFA-OPC₇₅₀ exhibited an ultrahigh CO₂ capture capacity of 26.6 mmol·g⁻¹ (117 wt %), outperforming other doped PCs by a large margin. Moreover, OPC₇₅₀ adsorbed slightly, but repeatedly, more CO₂ than the recently reported activated PCs made from asphalt Versatrol-HT (26.6 versus 26 mmol·g⁻¹).

It should be noted that the latter material required multiple activation steps, pretreatment, N-addition and reduction by H₂ to be capable of adsorbing such quantity of CO₂, which is in contrast with the far simpler process described herein. In contrast, OPC₇₅₀ is prepared from a simple precursor in a single activation step without N-addition or H₂ reduction.

To the best of Applicants' knowledge, among the category of high CO₂ uptake PC materials, FFA-OPC₇₅₀ exhibits remarkable CO₂ capture properties comparable to expensive MOFs with similar apparent surface area. For example, IRMOF-1 with a surface area of 2833 m²g⁻¹ captures about 21 mmol·g⁻¹ of CO₂ at 30 bar. IRMOF-6 with a surface area of 2516 m²g⁻¹ captures about 19 mmol·g⁻¹ at 30 bar. The overall volumetric CO₂ uptake results at 30 and 10 bar, for various sorbents are compared in Table 7, while the maximum amounts of gas uptakes at 30 bar for different PC samples with corresponding surface area is represented by FIG. 37.

For large scale gas uptake applications, such as removing CO₂ from natural gas, it is essential for a solid sorbent to exhibit both reproducible gas uptake capability and batch to batch reproducibility. This important requirement was examined via two experiments as shown in FIGS. 18A-18B, which displays a set of pressure dependent CO₂ uptake plots for different OPC₇₅₀ batches synthesized and activated same way. Almost identical gas adsorption characteristics, up to an upper pressure limit of 30 bar, confirmed the applicability of both OPC materials as cheap but very effective sorbent for industrial application. The other necessity for practical usage was further established by multiple cycles of gas adsorption-desorption measurements (2 cycles of adsorption and 2 cycles of desorption) that showed negligible or no hysteresis (FIG. 38C). Applicants' multiple cycle measurements and prolonged exposure to CO₂ on the same OPC sample satisfied two major requirements for practical application: no degradation in quality and no drop in gas uptake capacity.

For most of the solid sorbents, the gas uptake capacity increases with decreasing capture temperature. At 0.5° C. and a pressure of 30 bar, FFA-OPC₇₅₀ demonstrated an ultrahigh CO₂ uptake capacity that maxed to 189 wt % (43 mmol·g⁻¹) that is 60% higher than room temperature uptake (FIG. 39A). For any solid porous sorbents with high surface area, the trend of gas uptake is: the higher the gas pressure, the higher the CO₂ uptake and the uptake is both pressure and temperature dependent.

This result is further established by a set of plots describing the gas uptake capacity at four different uptake pressures as a function of experiment temperature (FIG. 39B). At a pressure of 5 bar, the CO₂ uptake varied from 5.2 to 12.6 mmol·g⁻¹ (increased by 142%) for a temperature change of 60 to 0.5° C.; whereas, at 30 bar, the change was significantly high, uptake varied from 12.6 to 42.9 mmol·g⁻¹ (increased by 240%). This important result signifies the possibility of selective CO₂ removal by exploiting the pressure-temperature dependent adsorption and desorption from a CO₂ rich gas mixture.

Example 3.8. CO₂/CH₄ Selectivity

The selective removal of CO₂ from natural gas, which essentially contains CH₄ and higher hydrocarbons along with other gases (CO₂, H₂S, and N₂), is one of the important industrial research goals, because these contaminant gases decrease power efficiency of the natural gas. The capture of CO₂ from natural gas primarily relies on purification strategies that allow the gas mixture to pass through a column packed with solid porous materials that captures CO₂ from the CH₄-rich environment with minimal CH₄ uptake. Applicants have previously shown that, unlike total CO₂ adsorption, the best CH₄:CO₂ adsorption ratio requires a PC with a surface area of more than 2000 m²g⁻¹, a total pore volume of more than 1.0 cm³g⁻¹, and a carbon content of less than 90 wt %. Based upon the forgoing, both OPC₇₅₀ materials meet these requirements. The absolute CO₂:CH₄ selectivity test was carried out by measuring volumetric CO₂ and CH₄ uptake isotherms up to a high pressure limit of 30 bar at 0.5 and 24.0° C. Applicants' study focused on the selectivity of FFA-OPC₇₅₀ and AA-OPC₇₅₀.

Two sets of volumetric CO₂ and CH₄ adsorption uptake measurements performed on each OPC₇₅₀ sorbent, at 0.5 and 24.0° C. (FIGS. 40A-40B, respectively). A similar set of room temperature uptake result for activated charcoal are presented in FIG. 40C. Here, the molar uptake selectivity (η_(CO2)/η_(CH4)) is defined by the molar ratio of adsorbed CO₂ and CH₄ at a certain pressure (i.e., at 30 bar).

Although the surface are and pore volumes and the CO₂ uptake of the two different OPC samples appear to be essentially independent of the choice of precursor (see FIGS. 29A-29B and Table 7), the same is not true of CH₄ uptake. As may be seen from Table 7, the CH₄ uptake for FFA-OPC is greater than that for AA-OPC for any given activation temperature. Given the relationship observed in FIGS. 29A-29B, this trend is also true for surface area and pore volume (FIGS. 41A-41B). Thus, even with similar physical parameters (surface area and pore volume) OPC prepared from PFFA shows greater CH₄ uptake than materials prepared from PAA. Although the differences are about 10%, this results in a comparable difference in CO₂/CH₄ selectivity, with AA-OPC samples providing better selectivity than FFA-OPC samples across the range of activation temperatures (Table 7).

Another important parameter that can be determined from FIGS. 42A-42D is the corresponding isosteric heat of adsorption of CO₂ and CH₄ for OPC₇₅₀ using the thermodynamic equations described elsewhere. In thermodynamic point of view, isosteric heat of adsorption of a gas determines the temperature change in a sorbent as a result of adsorption of adsorbate molecules to the sorbent surface and thus, it is one of the key thermodynamic parameters that can be utilized to separate this gas from a mixture of gases. The higher the difference between isosteric heats of adsorption for two gases the better will be the separation. For instance, as shown by FIGS. 42A-42D, there is a higher value for the isosteric heat of adsorption of CO₂ as compared CH₄ (Table 9), which allows Applicants to propose a temperature dependent strategy for removing CO₂ from natural gas via selective adsorption and desorption of CH₄ and CO₂. The results also suggest that understanding the factors controlling the difference between the values will offer a guide to the design of future adsorbents.

TABLE 9 Isosteric heat of adsorption for CO₂ and CH₄. Sample CO₂ (kJ mol⁻¹) CH₄ (kJ mol⁻¹) FFA-OPC₇₅₀ 23 13 AA-OPC₇₅₀ 33 14

Applicants have previously reported that with regard to CO₂ uptake, the relative distribution of pores within defined ranges defined performance. The micro- and meso-porosity analysis of these samples was determined by the t-plot method and revealed pore volume dependencies on KOH amounts (Table 10). The absolute volumes and percentage of total pore volume for OPC₇₅₀ samples as a function of the CO₂/CH₄ selectivity suggest that Applicants' previous proposal is correct. The greater the relative percentage of pores less than 2 nm, the greater the CO₂ uptake is in line with previous suggestions. However, the comparison of FFA-OPC₇₅₀ and AA-OPC₇₅₀ provides further insight into Applicants' previous proposal that it was pores in the range of 1-2 nm that are most important in defining CO₂/CH₄ selectivity.

In order to recognize the PC sorbent with the highest CO₂/CH₄ selectivity, Applicants surveyed molar selectivity (at 30 bar) of recently explored PC sorbents such as SPC, NPC and activated charcoal and AA-OPC₇₅₀. The absolute molar (CO₂/CH₄) uptake selectivity of OPC₇₅₀ (3.05) is greater than values for SPC (2.6), reduced-NPC (2.2) and activated charcoal (1.4) and slightly higher than the recently reported asphalt Versatrol-HT derived PC (3.0). These results clearly established that among the category of activated PC materials for selective CO₂ capture from natural gas, AA-OPC₇₅₀ is one of the best absorbents reported so far and much lower cost relative to SPC and NPC prepared from analogous polymers.

TABLE 10 Summary of meso (>2 nm), micro (<2 nm), and narrower micropore (<1 nm) volume (V) for OPC₇₅₀ samples. Micropore volumes determined by the t-plot method. Within parentheses, % of the total pore volume is shown. V_(MICRO) V_(NARROW) V_(i) V_(MESO) V_(MACRO) (0-2 nm) (0-1 nm) (1-2 nm) (2-50 nm) (>50 nm) Sample^(a) (cm³g⁻¹) (cm³g⁻¹) (m²g⁻¹) (cm³g⁻¹) (m²g⁻¹) FFA-OPC₇₅₀ 1.10 0.23 (13%) 0.87 (49%) 0.57 (32%) 0.10 (6%) AA-OPC₇₅₀ 1.24 0.12 (6%)  1.12 (60%) 0.58 (31%) 0.05 (3%)

Based upon prior work, it is known that the best CO₂ uptake and CO₂/CH₄ differentiation is obtained with a defined set of parameters involving surface area, pore volume, and carbon content. The latter has been related to the relative percentage of pores less than 2 nm. These results suggested that the way to prepare an ideal PC adsorbent is to use a pre-formed O-containing precursor, and the formation of both PFFA and PAA meets these needs. Thus, the use of a designed precursor allows for the reproducible formation of an OPC material with the required physical attributes. Furthermore, unlike NPC and SPC materials, the OPC reported herein lends to pellet formation as required for scalable processes.

In this Example, the structural features of the precursor appear to be irrelevant to the OPC that is formed when considering CO₂ adsorption. However, this is not true for CH₄ adsorption and hence CO₂/CH₄ selectivity. Based upon the results herein, and without being bound by theory, Applicants suggest that the identity of the precursor and the subsequent control over the pore structure is important for CH₄ adsorption and hence CO₂/CH₄ selectivity. In conclusion, Applicants propose in this Example that, while CO₂ uptake is optimized by maximization of pores of less than 2 nm, the CO₂/CH₄ selectivity requires optimization of pores in the 1-2 nm range.

Example 4. The Effect of KOH Concentration in Chemical Activation of Porous Carbon Sorbents for Carbon Dioxide Uptake and Carbon Dioxide-Methane Selectivity: The Relative Formation of Micro (<2 nm) Versus Meso (>2 nm) Porosity

In this Example, Applicants demonstrate that PC sorbents are synthesized from polymer precursors mixed with a chemical activation reagent and pyrolyzed (>500° C.). KOH is known to be the best activator for a wide range of precursors, as it creates PCs with a large surface area (1200-4000 m²g⁻¹). In order to determine the optimum KOH:polymer ratio for both CO₂ adsorption and CO₂/CH₄ selectivity, Applicants prepared a set of five S-containing porous carbon (SPC) samples from polythiophene (PTh) with increasing KOH:PTh ratio (1 to 5), and investigated CO₂ and CH₄ uptake measurements on carbonaceous SPC samples up to a pressure limit of 30 bar. The SPCs have been characterized by XPS, SEM, TEM and BET surface area analysis.

Although the apparent surface area and total pore volume increased with increasing KOH concentration, the maximum CO₂ uptake (5-30 bar) was demonstrated for samples with KOH:PTh=3. This equates to SPC samples with a surface area and total pore volume of ˜2700 m²g⁻¹ and 1.5 cm³g⁻¹, respectively. Greater values for either parameter do not enhance the CO₂ uptake, showing that it is not total porosity that is important. SPC samples formed with KOH:PTh=3 show both a maximum C composition (85%), and a maximum fraction of micro (<2 nm) porosity with a concomitant decrease in meso-pores (>2 nm). KOH:PTh=3 is also the synthetic conditions to maximize CH₄ uptake (5-30 bar). However, the optimum CO₂/CH₄ selectivity occurred with KOH:PTh=2. This correlates with different surface area (2,200 m²g⁻¹) and total pore volume (1.2 cm³g⁻¹) that required for optimum CO₂ uptake.

These results suggest that process conditions that lead to high relative micro porosity need to be considered rather than total surface area or pore volume. These results also suggest that, besides surface area and total pore volume of a particular sample, the relative composition of meso and micro porosity is the defining structural feature for optimizing CO₂ uptake.

Example 4.1. Materials and Methods

FeCl₃, 2-thiophenemethanol (purchased from Sigma Aldrich, 98% purity), CH₃CN, powdered KOH, distilled water, acetone, HCl, Ar (99.9% pure), CO₂ (99.99% pure, Matheson TRIGAS) and CH₄ (99.9% pure) were used as supplied. Polymer precursors and SPC materials were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and BET surface area analysis. The XPS measurements were carried out in a PHI Quantera scanning XPS microprobe. The wt % of chemical elements was determined by XPS survey scans with pass energy of 140 eV. For detailed elemental analysis, high-resolution multi-cycle elemental scans with pass energy 26 eV was performed. Each spectrum was then deconvoluted by appropriate basis functions. Before spectral fitting, each spectrum was corrected for reference binding energy for C1s to 284.8 eV. FTIR spectral measurements were performed in a Nicolet FTIR Infrared Microscope equipped with a liquid N₂ cooled detector. Scanning electron microscopic images were obtained by a FEI Quanta 400 ESEM FEG high-resolution field emission scanning electron microscope. The high-resolution TEM images of activated SPCs were obtained by a JEOL 2100 field emission gun transmission electron microscope.

The textural properties (i.e., surface areas, distributions of pore volumes and total pore volume) of carbonaceous materials were obtained by analyzing N₂ sorption isotherms (measured at 77 K), measured in a Quantachrome Autosorb-3b BET Surface Analyzer. Before measurements, samples were dried at 130° C. for 6 hours under high vacuum system equipped with a liquid N₂ cold trap. The apparent BET surface area (SBET) was calculated from N₂ adsorption isotherms in the partial pressure (P/P0) range of 0.05-0.30 by the multipoint BET (Brunauer-Emmett-Teller) method. The total pore volume was estimated from the amount of adsorbed N₂ at P/P₀=0.99. The distributions of pore volumes were determined by analyzing the data via non-local density functional theory. Pore volumes and surface area of micropores were determined by analyzing N₂ isotherms by t-plot method. Results are given in Table 11.

TABLE 11 Summary of PC and SPC samples studied with their elemental analysis, physical properties, and CO₂ uptakes. Total pore CO₂ uptake C O S Surface area volume V_(p) CO₂ uptake at at 30 bar Sample^(a) (wt %)^(b) (wt %)^(b) (wt %)^(b) S_(BET) (m²g⁻¹) (cm³g⁻¹) 30 bar (mmol · g⁻¹) (mg · g⁻¹) Act. 94.10 5.90 0.00 845 0.47 8.45 372 charcoal^(c) BPL^(d) 91.30 8.70 0.00 951 0.53 8.66 381 SPC-1 76.96 14.30 8.74 1680 0.98 14.68 646 SPC-2 78.89 13.73 7.37 2180 1.22 19.18 844 SPC-3 85.53 13.70 0.77 2675 1.51 22.64 996 SPC-4 84.63 15.37 0.00 2860 1.66 21.82 960 SPC-5 84.38 15.62 0.00 2980 1.76 21.29 937 PTh 61.45 5.39 29.49 40 0.02 2.40 106 ^(a)PC-KOH:precursor ratio. All SPC samples were activated at 700° C. ^(b)Determined by XPS. ^(c)Purchased from Mallinckrodt chemical works. ^(d)Purchased from Calgon carbon corp. Total pore volumes are measured at P/P₀ ~0.99.z

Example 4.2. Synthesis of S-Containing Polymer Precursor (PTh)

The polymer precursor was prepared by a modification of previously reported protocols. A solution of 2-thiophenemethanol (5 g, Sigma Aldrich, 98% purity) mixed with CH₃CN (20 mL) was slowly added to a solution of FeCl₃ (25 g) in CH₃CN (200 mL). The mixture was stirred for 2 hours under continuous Ar purging. The brown polythiophene (PTh) was separated by filtration, washed with DI water (4 L) and acetone (1 L), and dried at 60° C. for 12 hours under vacuum (Yield=98%).

Example 4.3. Conversion of PTh to S-Containing Porous Carbon (SPC)

In a typical activation process, PTh (500 mg) was thoroughly mixed with KOH powder (crushed previously, with KOH:PTh weight ratio varying from 1 to 5) in a mortar for 10 minutes. The mixture was then placed inside a quartz tube/tube furnace, dried for 10 minutes and then heated for 1 hour at a stable temperature of 700° C., under a flow of Ar (99.9%, flow rate 600 sccm). The activated samples were then washed with DI water, HCl (100 mL, 1.4 M) to remove excess inorganic salt residue and DI water until the filtrate attained pH=7. The product was dried at 80° C. for 12 hours under vacuum.

Example 4.4. CO₂ and CH₄ Uptake Measurements

The volumetric uptake measurements (pressure dependent excess isotherms) of CO₂ and CH₄ were performed in an automated Sievert instrument (Setaram PCTPRO). Various OPC samples were first crushed into powders and packed in a stainless steel autoclave sample cell. Initial sample pre-treatment was carried out at 130° C. for 1.5 hours under high vacuum. The free volume inside the sample cell was determined by a series of calibration procedures done under helium. Gas uptake experiments were carried out with high purity research grade CO₂ (99.99% purity, Matheson TRIGAS) and CH₄ (99.9% purity). A summary of selected results is given in Table 11.

Example 4.5. Results and Discussion

The polymer precursor Applicants selected for the synthesis of activated sulfur containing porous carbon (SPC) sorbents is poly[2-thiophenemethanol] (PTh) synthesized by reacting with FeCl₃ following the protocol reported elsewhere. This polymer was further activated by a strong oxidant, KOH, at a fixed temperature under inert atmosphere.

In general, the optimization procedure for synthesis of a PC sorbent with high surface area, from a polymer precursor activated by KOH, depends on two major parameters: finding the right KOH to PTh weight ratio; and the identification of the correct temperature of activation that gave satisfactory porosity and yield. Earlier reports suggest that the porosity of a SPC sorbent increased with activation temperature. However, a final yield of the activated product decreased significantly with activation temperature above 700° C. Additionally, previous reports suggest that the overall porosity and the surface area of a chemically activated PC material increase with KOH concentration.

Therefore, Applicants synthesized a set of SPC samples activated at a fixed activation temperature with gradually increasing KOH:PTh ratio r, where r varies from 1 to 5. These samples are labelled by SPC-r (Table 11).

The structural and textural morphology of synthesized PTh and activated SPC samples were characterized by scanning electron microscopy (SEM). The precursor demonstrated more rigid rock like blunt texture (FIG. 43A), while the SEM image of a SPC-2 sample (FIG. 43B) exhibits a texture full of micron size holes, multiple corners and edges that are absent in the precursor. The energy dispersive X-ray spectroscopy (EDS) confirmed that the SPCs are primarily composed of carbon, oxygen and sulfur.

Example 4.6. CO₂ Uptake

The volumetric CO₂ excess uptake (mmol of adsorbed CO₂ per g of sample) measurements for the SPC sorbent specimens activated at 700° C. with different KOH:PTh weight ratio are shown in FIG. 44 as a function of adsorbate pressure for the labelled SPC specimens, PTh and commercial charcoal powders (Mallinckrodt Chemical Works). In this set of isotherms, the C-precursor PTh adsorbed the least amount of CO₂ and the SPC-1 specimen demonstrated twice as much CO₂ adsorption as activated charcoal at 30 bar.

The difference in the shape of uptake isotherms confirms that gas uptake strongly depends on the KOH:PTh weight ratio (r). In particular, higher values of r correlate with higher uptake amounts for a specific adsorbate pressure (>12 bar up to r=3). Moreover, the SPC-4 and SPC-5 samples captured less CO₂ than SPC-3. The reproducibility of both the synthesis and the measurements is shown by a comparison of the volumetric CO₂ excess uptake of two batches of SPC-4 (FIG. 45).

Additional information for the dependence of gas uptake amounts at a specific pressure on the KOH:PTh weight ratio is presented by FIG. 46. The CO₂ capturing capacity increased from r=1 to 3 and then decreased again at higher r values. Previous low pressure studies (1 bar) results for KOH activation of petroleum coke show that KOH ratio of 3 and 4 show similar results.

Decreased uptake for SPC-4 and SPC-5 would have been expected to be a consequence of decreased surface area and/or pore volume. However, as seen from FIGS. 47A-47B, such expectation is not true. Instead, above a surface area value of 2675 m²g⁻¹ and total pore volume of 1.51 cm³g⁻¹, the gas uptake began to drop. This indicates that there are other factors besides surface area and pore volume that influence the CO₂ uptake capacity of a specific SPC sample.

The chemical composition of the SPCs activated with different amounts of KOH was determined by X-ray photoelectron spectroscopy (XPS) and compared with PTh and commercial activated carbons. Applicants note that XPS only provides surface (and near surface) chemical composition that may differ from bulk chemical composition. However, surface composition is what matters in a surface adsorption process.

It should also be noted that the H content is not provided by XPS data. Therefore, percentage values measured by other techniques will vary. The wt % of elements present as determined by XPS survey scans is presented in Table 11. The PTh and consequently the SPC-r samples were primarily composed of C, O, and S and chemical activation by increasing amount KOH gradually changed wt % of all three elements. FIGS. 48A-48B depict these changes.

Applicants have previously determined that for a wide range of PCs, the maximum CO₂ uptake occurs when the C composition (as determined by XPS) is in the range 80-95 wt %. In this Example, this range may be further specified as being more than 85%. However, the most important observation is that the trend observed in FIGS. 48A-48B is essentially the same as in FIG. 46. In particular, as indicated in FIG. 49, the CO₂ uptake increases with percentage of carbon content rather than the expected relationship with surface area or pore volume.

By consideration of the change in S content with increasing KOH:PTh ratio, it is clear that the loss of the majority of S correlates with the formation of PC samples with the highest CO₂ uptake (e.g., comparison between FIGS. 46 and 48B). However, replacement of S with O (at high KOH:PTh ratios) decreases the CO₂ uptake. Clearly, there is some significant physical change that occurs in these two regimes. In order to determine if there are distinct structural features that are associated with the C % and hence CO₂ uptake, Applicants have determined the pore structure changes that occur with KOH:PTh ratio.

The surface area, total pore volume and pore size distribution of SPC samples activated with different KOH:PTh ratios were determined by measuring low temperature (77 K) N₂ adsorption isotherms in a BET (Brunauer-Emmett-Teller) surface area analyzer. FIG. 50 shows such set of isotherms for five SPC-r samples activated at 700° C. Here, KOH:PTh ratio dependent differences in the shape of these isotherms was noticed. The isotherms for SPC-3 to SPC-5 are much steeper than SPC-1 or SPC-2 in the relative pressure range 0.05-0.3, defining rapid increase of surface area and adsorption capacity with higher amount of KOH.

The estimated surface area (S_(BET)) and the total pore volume (V_(p)) gradually increased with activation temperature (FIGS. 51A and 51B, respectively), describing the incremental trend for mildly to strongly activation conditions. As expected, the surface area increases with increased KOH:PTh ratio, although there is a change in the relationship above KOH:PTh=3. A similar trend is observed for the total pore volume (FIG. 51B), and the two show a near linear relationship. These demonstrate the strong influence of KOH on the porous properties of activated samples.

One key piece of information that can be obtained from the BET surface area analysis is the pore size distributions as a function of pore sizes of a specific porous solid. FIG. 52 plots pore size distribution as a function of pore size for a set of five SPCs activated with mild (KOH:PTh=1) to strong (KOH:PTh=5) activation conditions. These plots show that SPC-1 sample primarily contains pores narrower than ˜2 nm. As the KOH amount increases, wider pores began to form as evidenced by the PSD plot for SPC-2.

Samples activated with minimal amount of KOH (i.e., SPC-1) contained pores in the range of 1-3 nm. In contrast, the distribution plots for SPC-4 and SPC-5 indicate that chemical activation with large amounts of KOH created some additional mesopores in the 3-4.5 nm range. The SPC-5 sample even contained pores larger than 4 nm. Confirmation of the pore sizes is obtained from high resolution transmission electron microscopy (HRTEM). For example, FIG. 53 displays an image of SPC-2, demonstrating randomly distributed micropores with dimension in the range of 1-2 nm.

It is clear from FIG. 51B that total pore volumes (sum of pore volumes of narrower micro (<1 nm), micro, meso and macro-sized pores) systematically increase with KOH:PTh ratio. In addition, volume change behaves differently for different sizes of pores. The micro- and meso-porosity analysis of these samples was determined by the t-plot method and revealed pore volume dependencies on KOH amounts (Table 12).

FIGS. 54A-54B depict absolute volumes and percentage of total pore volume for a set of five SPC samples as a function of the KOH:PTh ratio. In contrast to total pore volumes (FIG. 51B), the relative pore composition of micropores (defined as <2 nm) shows a marked increase with increased KOH:PTh ratio until SPC-3, above which the fraction of the total pore volume associated with these size regimes decreases (FIG. 54A). The mesopore composition shows the obverse trend. Interestingly, the micropore volumes show the strongest dependence on KOH.

TABLE 12 Summary of meso (>2 nm), micro (<2 nm), and narrower micropore (<1 nm) volume (V) for PC and SPC samples studied.. V_(MICRO) V_(NARROW) V_(i) V_(MESO) V_(MACRO) (0-2 nm) (0-1 nm) (1-2 nm) (2-50 nm) (>50 nm) Sample^(a) (cm³g⁻¹) (cm³g⁻¹) (m²g⁻¹) (cm³g⁻¹) (m²g⁻¹) Act. Charcoal^(b) 0.32 0.11 (23%) 0.21 (45%) 0.11 (24%) 0.04 (8%) BPL^(c) 0.38 0.13 (25%) 0.25 (47%) 0.12 (23%) 0.03 (5%) SPC-1 0.71 0.19 (19%) 0.52 (54%) 0.18 (18%) 0.09 (9%) SPC-2 0.76 0.16 (13%) 0.60 (49%) 0.35 (29%) 0.11 (9%) SPC-3 1.01 0.18 (12%) 0.83 (55%) 0.38 (25%) 0.12 (8%) SPC-4 0.87 0.16 (10%) 0.71 (43%) 0.64 (38%) 0.15 (9%) SPC-5 0.88 0.10 (6%)  0.78 (44%) 0.72 (41%) 0.16 (9%) ^(a)PC-KOH:precursor ratio. All SPC samples were activated at 700° C. ^(b)Purchased from Mallinckrodt chemical works. ^(c)Purchased from Calgon carbon corp. Micropore volumes are determined by t-plot method. Micropores include pores between 0.4 to 2 nm. Within parentheses, % of total pore volumes is shown.

The relationship between relative composition of the various pore sizes and the CO₂ uptake is shown in FIGS. 55A-55C. This clearly shows that, in order to maximize CO₂ uptake, activation conditions (in this case KOH:PTh ratio) should be chosen to maximize the relative narrower micro and micro pores (especially those between 1-2 nm) rather than requiring solely on ever increasing surface area or pore volume.

A comparison of FIG. 54B with FIG. 48A suggests that narrower micropore formation is associated with the increased C content. However, mesopore formation appears to be controlled by increased O content. Previous work with carbide-derived carbons (CDCs) and PCs (at 1 bar) suggests that it is the pore volume less than 1 nm that is important. In contrast, it has also been suggested that mesopores (>2 nm) are the most important. There has also been a proposal that the important sizes depend on the pressure used. Applicants' results clearly show that, at higher pressures (>10 bar) it is a larger set (1-2 nm) that is controlling CO₂ uptake.

Example 4.7. CO₂/CH₄ Selectivity

The selective removal of CO₂ from natural gas, which essentially contains CH₄ and other gases such as CO₂, H₂S, and N₂, is one of the important industrial research goals, because these contaminant gases decrease power efficiency of the natural gas. Thus, Applicants have been directed to explore selective CO₂ capture capacity of different carbon-based porous sorbents at different gas pressure and temperature. For an ideal sorbent for selective removal of CO₂ from natural gas, the sorbent should demonstrate significantly lower CH₄ uptake than CO₂. In order to compare high pressure CH₄ uptake capacities of five SPC samples with their CO₂ uptakes, Applicants measured room temperature volumetric CH₄ uptake of same set of samples under similar condition.

FIG. 56 represents a set of such uptake isotherms. In contrast to CO₂ uptake isotherms, Applicants see two distinct sets of isotherms. One set comprises SPC-1 and SPC-2 and the other for SPC-3, 4, and 5. This feature is further demonstrated by a set of plots showing dependence of CH₄ uptake on the KOH:PTh weight ratio of the corresponding sorbent at a specific capture pressure (FIG. 57). For example, at 30 bar, there was negligible difference in CH₄ uptakes of all three samples SPC-3 to 5 (˜9 mmol·g⁻¹), suggesting that the amount of KOH had much weaker effect on the CH₄ uptake property relative to than CO₂ uptake, though surface area and porosity had changed significantly.

The molar uptake selectivity (molar CO₂:CH₄ uptake ratio) for the SPC samples as a function of gas pressure is shown in FIG. 58A. The selectivity traces for SPC-1 to SPC-3 varied smoothly between 10 to 30 bar. The dependence of high pressure selectivity at 30 bar on the KOH:PTh ratios, surface area and total pore volume of the corresponding SPCs are presented in FIGS. 58B-58D, respectively. Surprisingly, the SPC-2 sample (with surface area=2180 m²g⁻¹ and total pore volume=1.22 cm³g⁻¹) demonstrated highest molar selectivity (2.68) at 30 bar, though SPC-3 exhibited the highest CO₂ uptake.

The shift from KOH:PTh ratio of 3 to 2 for optimum selectivity as opposed to uptake for both CO₂ and CH₄ is due to the relative shape of the uptake as a function of reagent ratios (e.g., comparison of FIGS. 46 and 57). While the CO₂ uptake increased uniformly with KOH:PTh ratio from 1 to 3, that for CH₄ is a step function. This suggests that, in determining optimum selectivity, it is important to understand the variations between CO₂ and CH₄ adsorption rather than the maximum for both.

Applicants' prior work has demonstrated that the best CO₂ uptake and CO₂/CH₄ differentiation is obtained with a defined set of parameters involving surface area, pore volume, and carbon content which are in turn a function of the polymer precursor and the process activation temperature. In this Example, Applicants extend this work and demonstrate that there is an optimum KOH:PTh ratio for activation of the SPC. The ratio for optimum CO₂ (and CH₄ uptake) is 3, which equates to SPC samples with a surface area and total pore volume of 2700 m²g⁻¹ and 1.5 cm³g⁻¹, respectively. In contrast, the ratio is 2 for the best CO₂/CH₄ differentiation, which produces a surface area of 2,200 m²g⁻¹ and total pore volume of 1.2 cm³g⁻¹.

Moreover, in this Example, Applicants demonstrate that the optimum CO₂ uptake is not for a material with the highest pore volume or surface area, but for the material with S_(BET)>2000 m²g⁻¹ and the highest percentage of a maximum fraction of narrower micro (<1 nm) and micro (<2 nm) porosity as compared to meso-pores (>2 nm). Increasing in the latter type of pores does increase both the surface area and pore volume, but not the CO₂ uptake.

Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present disclosure to its fullest extent. The embodiments described herein are to be construed as illustrative and not as constraining the remainder of the disclosure in any way whatsoever. While the embodiments have been shown and described, many variations and modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims, including all equivalents of the subject matter of the claims. The disclosures of all patents, patent applications and publications cited herein are hereby incorporated herein by reference, to the extent that they provide procedural or other details consistent with and supplementary to those set forth herein. 

What is claimed is:
 1. A material for CO₂ adsorption at pressures above 1 bar comprising: a porous material with a surface area of at least 2,800 m²/g, and a total pore volume of at least 1.35 cm³/g, wherein more than 70% of pores of the porous material have diameters of less than 2 nm as measured from N₂ sorption isotherms using the BET (Brunauer-Emmett-Teller) method, wherein the porous material has an oxygen content of more than about 7 wt % as measured by X-ray photoelectron spectroscopy, and wherein the porous material has a CO₂ adsorption capacity of more than about 100 wt %.
 2. The material of claim 1, wherein the porous material comprises a porous carbon material with a carbon content of between 80% and 95% as measured by X-ray photoelectron spectroscopy.
 3. The material of claim 2, wherein the porous carbon material is prepared by heating an organic polymer precursor or biological material in the presence of KOH, wherein the temperature of activation is between 700° C. and 800° C.
 4. The material of claim 3, wherein the organic polymer precursor or biological material comprises oxygen in a functional group.
 5. The material of claim 4, wherein the functional group comprises a furyl.
 6. The material of claim 5, wherein the organic polymer precursor polymerizes to form polyfurfuryl alcohol.
 7. The material of claim 6, wherein the polyfurfuryl alcohol is prepared by the polymerization of furfuryl alcohol with a catalyst.
 8. The material of claim 7, wherein the catalyst comprises iron(III) chloride.
 9. The material of claim 3, wherein the biological material is chosen from at least one of the following: sawdust and coconut husk.
 10. The material of claim 4, wherein the functional group comprises an anisyl.
 11. The material of claim 10, wherein the organic polymer precursor polymerizes to form polyanisyl alcohol.
 12. The material of claim 11, wherein the polyanisyl alcohol is prepared by the polymerization of anisyl alcohol with a catalyst.
 13. The material of claim 12, wherein the catalyst comprises a protic acid.
 14. The material of claim 1, wherein more than 80% of pores of the porous material have diameters of less than 2 nm.
 15. The material of claim 1, wherein the porous material has an oxygen content of more than about 10 wt % as measured by X-ray photoelectron spectroscopy.
 16. The material of claim 1, wherein the molar CO₂:CH₄ uptake ratio of the porous material is more than about
 2. 17. A material for the separation of CO₂ from natural gas at partial pressures of either component above 1 bar comprising: a porous material with a surface area of at least 2,200 m²/g, and a total pore volume of at least 1.00 cm³/g, wherein more than 50% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm as measured from N₂ sorption isotherms using the BET (Brunauer-Emmett-Teller) method, and wherein the porous material has a CO₂ adsorption capacity of more than about 100 wt %.
 18. The material of claim 17, wherein the porous material comprises a porous carbon material with a carbon content of between 80% and 95% as measured by X-ray photoelectron spectroscopy.
 19. The material of claim 18, wherein the porous carbon material is prepared by heating an organic polymer precursor or biological material in the presence of KOH, wherein the temperature of activation is between 700° C. and 800° C.
 20. The material of claim 19, wherein the organic polymer precursor comprises oxygen in a functional group.
 21. The material of claim 20, wherein the functional group comprises a furyl.
 22. The material of claim 21, wherein the organic polymer precursor polymerizes to form polyfurfuryl alcohol.
 23. The material of claim 22, wherein the polyfurfuryl alcohol is prepared by the polymerization of furfuryl alcohol with a catalyst.
 24. The material of claim 23, wherein the catalyst comprises iron(III) chloride.
 25. The material of claim 19, wherein the biological material is chosen from at least one of the following: sawdust and coconut husk.
 26. The material of claim 20, wherein the functional group comprises an anisyl.
 27. The material of claim 26, wherein the organic polymer precursor polymerizes to form polyanisyl alcohol.
 28. The material of claim 27, wherein the polyanisyl alcohol is prepared by the polymerization of anisyl alcohol with a catalyst.
 29. The material of claim 28, wherein the catalyst comprises a protic acid.
 30. The material of claim 17, wherein more than 60% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm.
 31. The material of claim 17, wherein the porous material has an oxygen content of more than about 7 wt % as measured by X-ray photoelectron spectroscopy.
 32. The material of claim 17, wherein the molar CO₂:CH₄ uptake ratio of the porous material is more than about
 2. 33. The material of claim 17, wherein the porous material has an oxygen content of more than about 10 wt % as measured by X-ray photoelectron spectroscopy.
 34. A material for the separation of CO₂ from natural gas at partial pressures of either component above 1 bar comprising: a porous material with a surface area of at least 2,200 m²/g, and a total pore volume of at least 1.00 cm³/g, wherein more than 40% of pores of the porous material have diameters of greater than 1 nm and less than 2 nm as measured from N₂ sorption isotherms using the BET (Brunauer-Emmett-Teller) method, and wherein the porous material has a CO₂ adsorption capacity of more than about 100 wt %. 